diff --git a/certs/certs.go b/certs/certs.go
index f6dfef59..fef67a69 100644
--- a/certs/certs.go
+++ b/certs/certs.go
@@ -46,7 +46,7 @@ type FinalityCertificate struct {
 	Signature []byte
 	// Changes between the power table used to validate this finality certificate and the power
 	// used to validate the next finality certificate. Sorted by ParticipantID, ascending.
-	PowerTableDelta PowerTableDiff `json:"omitempty"`
+	PowerTableDelta PowerTableDiff `json:"PowerTableDelta,omitempty"`
 }
 
 // NewFinalityCertificate constructs a new finality certificate from the given power delta (from
diff --git a/gpbft/chain_test.go b/gpbft/chain_test.go
index bbf4c179..17b273f2 100644
--- a/gpbft/chain_test.go
+++ b/gpbft/chain_test.go
@@ -188,7 +188,7 @@ func TestECChain_Eq(t *testing.T) {
 	}
 }
 
-func TestTipSetKeySerialization(t *testing.T) {
+func TestTipSetSerialization(t *testing.T) {
 	t.Parallel()
 	var (
 		c1        = gpbft.MakeCid([]byte("barreleye1"))
@@ -268,6 +268,11 @@ func TestTipSetKeySerialization(t *testing.T) {
 			for j, c := range []cid.Cid{c1, c2, c3}[:len(ts.Key)/38] {
 				req.Equal(map[string]any{"/": c.String()}, keyField[j])
 			}
+
+			// check that the supplemental data is a base64 string
+			commitField, ok := bareMap["Commitments"].(string)
+			req.True(ok)
+			req.Len(commitField, 44)
 		}
 	})
 
diff --git a/gpbft/gpbft.go b/gpbft/gpbft.go
index 37ab5bb3..8918cd5b 100644
--- a/gpbft/gpbft.go
+++ b/gpbft/gpbft.go
@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"encoding/binary"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"math"
@@ -94,6 +95,35 @@ func (d *SupplementalData) Eq(other *SupplementalData) bool {
 	return d.Commitments == other.Commitments && d.PowerTable == other.PowerTable
 }
 
+// Custom JSON marshalling for SupplementalData to achieve a commitment field
+// that is a base64-encoded string.
+
+type supplementalDataSub SupplementalData
+type supplementalDataJson struct {
+	Commitments []byte
+	*supplementalDataSub
+}
+
+func (sd SupplementalData) MarshalJSON() ([]byte, error) {
+	return json.Marshal(&supplementalDataJson{
+		Commitments:         sd.Commitments[:],
+		supplementalDataSub: (*supplementalDataSub)(&sd),
+	})
+}
+
+func (sd *SupplementalData) UnmarshalJSON(b []byte) error {
+	aux := &supplementalDataJson{supplementalDataSub: (*supplementalDataSub)(sd)}
+	var err error
+	if err = json.Unmarshal(b, &aux); err != nil {
+		return err
+	}
+	if len(aux.Commitments) != 32 {
+		return errors.New("commitments must be 32 bytes")
+	}
+	copy(sd.Commitments[:], aux.Commitments)
+	return nil
+}
+
 // Fields of the message that make up the signature payload.
 type Payload struct {
 	// GossiPBFT instance (epoch) number.
diff --git a/gpbft/gpbft_test.go b/gpbft/gpbft_test.go
index 4bde70fa..afeeb3e0 100644
--- a/gpbft/gpbft_test.go
+++ b/gpbft/gpbft_test.go
@@ -3,6 +3,7 @@ package gpbft_test
 import (
 	"bytes"
 	"crypto/rand"
+	"encoding/json"
 	"io"
 	"testing"
 
@@ -1734,3 +1735,57 @@ func TestGPBFT_Sway(t *testing.T) {
 		require.Fail(t, "after 10 tries did not swayed to proposals 1 and 2 at CONVERGE and COMMIT, respectively.")
 	})
 }
+
+func TestSupplementalDataSerialization(t *testing.T) {
+	t.Parallel()
+	var (
+		testCases = []gpbft.SupplementalData{
+			{
+				PowerTable:  gpbft.MakeCid([]byte("fish")),
+				Commitments: [32]byte{0x01},
+			},
+			{
+				PowerTable:  gpbft.MakeCid([]byte("lobster")),
+				Commitments: [32]byte{0x02},
+			},
+		}
+	)
+
+	t.Run("cbor round trip", func(t *testing.T) {
+		req := require.New(t)
+		for _, ts := range testCases {
+			var buf bytes.Buffer
+			req.NoError(ts.MarshalCBOR(&buf))
+			t.Logf("cbor: %x", buf.Bytes())
+			var rt gpbft.SupplementalData
+			req.NoError(rt.UnmarshalCBOR(&buf))
+			req.Equal(ts, rt)
+		}
+	})
+
+	t.Run("json round trip", func(t *testing.T) {
+		req := require.New(t)
+		for _, ts := range testCases {
+			data, err := ts.MarshalJSON()
+			req.NoError(err)
+			t.Logf("json: %s", data)
+			var rt gpbft.SupplementalData
+			req.NoError(rt.UnmarshalJSON(data))
+			req.Equal(ts, rt)
+
+			// check that the supplemental data is a base64 string
+			var bareMap map[string]any
+			req.NoError(json.Unmarshal(data, &bareMap))
+			commitField, ok := bareMap["Commitments"].(string)
+			req.True(ok)
+			req.Len(commitField, 44)
+		}
+	})
+
+	t.Run("json error cases", func(t *testing.T) {
+		req := require.New(t)
+		var ts gpbft.SupplementalData
+		err := ts.UnmarshalJSON([]byte(`{"Commitments":"bm9wZQ==","PowerTable":{"/":"bafy2bzaced5zqzzbxzyzuq2tcxhuclnvdn3y6ijhurgaapnbayul2dd5gspc4"}}`))
+		req.ErrorContains(err, "32 bytes")
+	})
+}