diff --git a/avogadro/io/mdlformat.cpp b/avogadro/io/mdlformat.cpp
index fe41c111d9..66fc36924f 100644
--- a/avogadro/io/mdlformat.cpp
+++ b/avogadro/io/mdlformat.cpp
@@ -23,6 +23,7 @@ using Avogadro::Core::Bond;
 using Avogadro::Core::Elements;
 using Avogadro::Core::lexicalCast;
 using Avogadro::Core::Molecule;
+using Avogadro::Core::split;
 using Avogadro::Core::startsWith;
 using Avogadro::Core::trimmed;
 
@@ -92,8 +93,10 @@ bool MdlFormat::read(std::istream& in, Core::Molecule& mol)
     return false;
   }
   string mdlVersion(trimmed(buffer.substr(33)));
-  if (mdlVersion != "V2000") {
-    appendError("Unsupported file format version encountered: " + mdlVersion);
+  if (mdlVersion == "V3000")
+    return readV3000(in, mol);
+  else if (mdlVersion != "V2000") {
+    appendError("Unsupported MDL version: " + mdlVersion);
     return false;
   }
 
@@ -241,8 +244,8 @@ bool MdlFormat::read(std::istream& in, Core::Molecule& mol)
         dataValue += buffer;
       }
     } else if (startsWith(buffer, "> <")) {
-      // This is a data header, read the name of the entry, and the value on the
-      // following lines.
+      // This is a data header, read the name of the entry, and the value on
+      // the following lines.
       dataName = trimmed(buffer).substr(3, buffer.length() - 4);
       inValue = true;
     }
@@ -251,11 +254,207 @@ bool MdlFormat::read(std::istream& in, Core::Molecule& mol)
   return true;
 }
 
+bool MdlFormat::readV3000(std::istream& in, Core::Molecule& mol)
+{
+  string buffer;
+  // we should have M  V30 BEGIN CTAB
+  getline(in, buffer);
+  if (trimmed(buffer) != "M  V30 BEGIN CTAB") {
+    appendError("Error parsing V3000 file, expected 'M  V30 BEGIN CTAB'.");
+    return false;
+  }
+  // now we should get the counts line
+  // e.g. 'M  V30 COUNTS 23694 24297 0 0 1'
+  getline(in, buffer);
+  // split by whitespace
+  std::vector<string> counts = split(trimmed(buffer), ' ');
+  if (counts.size() < 5) {
+    appendError("Error parsing V3000 counts line.");
+    return false;
+  }
+  bool ok(false);
+  int numAtoms(lexicalCast<int>(counts[3], ok));
+  if (!ok) {
+    appendError("Error parsing number of atoms.");
+    return false;
+  }
+  int numBonds(lexicalCast<int>(counts[4], ok));
+  if (!ok) {
+    appendError("Error parsing number of bonds.");
+    return false;
+  }
+
+  // Parse the atom block.
+  // 'M  V30 BEGIN ATOM'
+  // 'M  V30 1 N 171.646 251.874 224.877 0'
+  getline(in, buffer);
+  if (trimmed(buffer) != "M  V30 BEGIN ATOM") {
+    appendError("Error parsing V3000 atom block.");
+    return false;
+  }
+  for (int i = 0; i < numAtoms; ++i) {
+    getline(in, buffer);
+    std::vector<string> atomData = split(trimmed(buffer), ' ');
+    if (atomData.size() < 7) {
+      appendError("Error parsing V3000 atom line.");
+      return false;
+    }
+
+    string element(trimmed(atomData[3]));
+    unsigned char atomicNum = Elements::atomicNumberFromSymbol(element);
+    Atom newAtom = mol.addAtom(atomicNum);
+
+    Vector3 pos;
+    pos.x() = lexicalCast<Real>(atomData[4], ok);
+    if (!ok) {
+      appendError("Failed to parse x coordinate: " + atomData[3]);
+      return false;
+    }
+    pos.y() = lexicalCast<Real>(atomData[5], ok);
+    if (!ok) {
+      appendError("Failed to parse y coordinate: " + atomData[4]);
+      return false;
+    }
+    pos.z() = lexicalCast<Real>(atomData[6], ok);
+    if (!ok) {
+      appendError("Failed to parse z coordinate: " + atomData[5]);
+      return false;
+    }
+    newAtom.setPosition3d(pos);
+    // check for formal charge in the atom block
+    // CHG=1 for example
+    if (atomData.size() > 7) {
+      string chargeData = atomData[7];
+      if (startsWith(chargeData, "CHG=")) {
+        int charge = lexicalCast<int>(chargeData.substr(4), ok);
+        if (!ok) {
+          appendError("Failed to parse atom charge: " + chargeData);
+          return false;
+        }
+        mol.setFormalCharge(newAtom.index(), charge);
+      }
+    }
+  } // end of atom block
+  getline(in, buffer);
+  // check for END ATOM
+  if (trimmed(buffer) != "M  V30 END ATOM") {
+    appendError("Error parsing V3000 atom block.");
+    return false;
+  }
+
+  // bond block
+  // 'M  V30 BEGIN BOND'
+  // 'M  V30 1 1 1 2'
+  getline(in, buffer);
+  if (trimmed(buffer) != "M  V30 BEGIN BOND") {
+    appendError("Error parsing V3000 bond block.");
+    return false;
+  }
+  for (int i = 0; i < numBonds; ++i) {
+    getline(in, buffer);
+    std::vector<string> bondData = split(trimmed(buffer), ' ');
+    if (bondData.size() < 5) {
+      appendError("Error parsing V3000 bond line.");
+      return false;
+    }
+    int order = lexicalCast<int>(bondData[3], ok);
+    if (!ok) {
+      appendError("Failed to parse bond order: " + bondData[3]);
+      return false;
+    }
+    int atom1 = lexicalCast<int>(bondData[4], ok) - 1;
+    if (!ok) {
+      appendError("Failed to parse bond atom1: " + bondData[4]);
+      return false;
+    }
+    int atom2 = lexicalCast<int>(bondData[5], ok) - 1;
+    if (!ok) {
+      appendError("Failed to parse bond atom2: " + bondData[5]);
+      return false;
+    }
+    mol.addBond(mol.atom(atom1), mol.atom(atom2),
+                static_cast<unsigned char>(order));
+  } // end of bond block
+
+  // look for M  END
+  while (getline(in, buffer)) {
+    if (trimmed(buffer) == "M  END")
+      break;
+  }
+  // read in any properties
+  while (getline(in, buffer)) {
+    if (startsWith(buffer, "> <")) {
+      string key = trimmed(buffer.substr(3, buffer.length() - 4));
+      string value;
+      while (getline(in, buffer)) {
+        if (trimmed(buffer) == "")
+          break;
+        value += buffer + "\n";
+      }
+      mol.setData(key, value);
+    }
+  }
+
+  return true;
+}
+
+bool MdlFormat::writeV3000(std::ostream& out, const Core::Molecule& mol)
+{
+  // write the "fake" counts line
+  out << "  0  0  0     0  0            999 V3000\n";
+  out << "M  V30 BEGIN CTAB\n";
+  out << "M  V30 COUNTS " << mol.atomCount() << ' ' << mol.bondCount()
+      << " 0 0 0\n";
+  // atom block
+  out << "M  V30 BEGIN ATOM\n";
+  for (size_t i = 0; i < mol.atomCount(); ++i) {
+    Atom atom = mol.atom(i);
+    out << "M  V30 " << i + 1 << ' ' << Elements::symbol(atom.atomicNumber())
+        << ' ' << atom.position3d().x() << ' ' << atom.position3d().y() << ' '
+        << atom.position3d().z() << " 0";
+    if (atom.formalCharge())
+      out << " CHG=" << atom.formalCharge();
+    out << "\n";
+  }
+  out << "M  V30 END ATOM\n";
+  // bond block
+  out << "M  V30 BEGIN BOND\n";
+  for (size_t i = 0; i < mol.bondCount(); ++i) {
+    Bond bond = mol.bond(i);
+    out << "M  V30 " << i + 1 << ' ' << static_cast<int>(bond.order()) << ' '
+        << (bond.atom1().index() + 1) << ' ' << (bond.atom2().index() + 1)
+        << " \n";
+  }
+  out << "M  V30 END BOND\n";
+  out << "M  V30 END CTAB\n";
+  out << "M  END\n";
+
+  // TODO: isotopes, radicals, etc.
+  if (m_writeProperties) {
+    const auto dataMap = mol.dataMap();
+    for (const auto& key : dataMap.names()) {
+      out << "> <" << key << ">\n";
+      out << dataMap.value(key).toString() << "\n";
+      out << "\n"; // empty line between data blocks
+    }
+  }
+
+  if (m_writeProperties || isMode(FileFormat::MultiMolecule))
+    out << "$$$$\n";
+
+  return true;
+}
+
 bool MdlFormat::write(std::ostream& out, const Core::Molecule& mol)
 {
   // Header lines.
   out << mol.data("name").toString() << "\n  Avogadro\n\n";
   // Counts line.
+  if (mol.atomCount() > 999 || mol.bondCount() > 999) {
+    // we need V3000 support for big molecules
+    return writeV3000(out, mol);
+  }
+
   out << setw(3) << std::right << mol.atomCount() << setw(3) << mol.bondCount()
       << "  0  0  0  0  0  0  0  0999 V2000\n";
   // Atom block.
@@ -269,7 +468,7 @@ bool MdlFormat::write(std::ostream& out, const Core::Molecule& mol)
                                             : ((charge <= 3) ? charge : 0);
     out << setw(10) << std::right << std::fixed << setprecision(4)
         << atom.position3d().x() << setw(10) << atom.position3d().y()
-        << setw(10) << atom.position3d().z() << " " << setw(3) << std::left
+        << setw(10) << atom.position3d().z() << ' ' << setw(3) << std::left
         << Elements::symbol(atom.atomicNumber()) << " 0" << setw(3)
         << std::right << chargeField /* for compatibility */
         << "  0  0  0  0  0  0  0  0  0  0\n";
@@ -286,7 +485,7 @@ bool MdlFormat::write(std::ostream& out, const Core::Molecule& mol)
   for (auto& i : chargeList) {
     Index atomIndex = i.first;
     signed int atomCharge = i.second;
-    out << "M  CHG  1 " << setw(3) << std::right << atomIndex + 1 << " "
+    out << "M  CHG  1 " << setw(3) << std::right << atomIndex + 1 << ' '
         << setw(3) << atomCharge << "\n";
   }
   // TODO: isotopes, etc.
@@ -301,7 +500,7 @@ bool MdlFormat::write(std::ostream& out, const Core::Molecule& mol)
     }
   }
 
-  if (isMode(FileFormat::MultiMolecule))
+  if (m_writeProperties || isMode(FileFormat::MultiMolecule))
     out << "$$$$\n";
 
   return true;
diff --git a/avogadro/io/mdlformat.h b/avogadro/io/mdlformat.h
index c887221f61..1ae68d2173 100644
--- a/avogadro/io/mdlformat.h
+++ b/avogadro/io/mdlformat.h
@@ -52,7 +52,9 @@ class AVOGADROIO_EXPORT MdlFormat : public FileFormat
   std::vector<std::string> mimeTypes() const override;
 
   bool read(std::istream& in, Core::Molecule& molecule) override;
+  bool readV3000(std::istream& in, Core::Molecule& molecule);
   bool write(std::ostream& out, const Core::Molecule& molecule) override;
+  bool writeV3000(std::ostream& out, const Core::Molecule& molecule);
 
 protected:
   bool m_writeProperties = false;
diff --git a/avogadro/io/sdfformat.cpp b/avogadro/io/sdfformat.cpp
index 4238a13d9c..ce8761aa91 100644
--- a/avogadro/io/sdfformat.cpp
+++ b/avogadro/io/sdfformat.cpp
@@ -28,6 +28,7 @@ std::vector<std::string> SdfFormat::fileExtensions() const
 {
   std::vector<std::string> ext;
   ext.emplace_back("sdf");
+  ext.emplace_back("sd3");
   return ext;
 }