diff --git a/consensus/vote_set.go b/consensus/vote_set.go
index 62501fa1d..98c24080b 100644
--- a/consensus/vote_set.go
+++ b/consensus/vote_set.go
@@ -51,6 +51,7 @@ func NewVoteSet(height uint32, round uint16, type_ byte, vset *ValidatorSet) *Vo
 
 // True if added, false if not.
 // Returns ErrVote[UnexpectedPhase|InvalidAccount|InvalidSignature|InvalidBlockHash|ConflictingSignature]
+// NOTE: vote should be mutated after adding.
 func (vs *VoteSet) Add(vote *Vote) (bool, error) {
 	vs.mtx.Lock()
 	defer vs.mtx.Unlock()
diff --git a/consensus/vote_set_test.go b/consensus/vote_set_test.go
index c8db9e670..f7274ba8b 100644
--- a/consensus/vote_set_test.go
+++ b/consensus/vote_set_test.go
@@ -2,6 +2,7 @@ package consensus
 
 import (
 	. "github.com/tendermint/tendermint/blocks"
+	. "github.com/tendermint/tendermint/common"
 	. "github.com/tendermint/tendermint/state"
 
 	"testing"
@@ -16,7 +17,7 @@ func makeValidator(id uint64, votingPower uint64) (*Validator, *PrivAccount) {
 	}, privAccount
 }
 
-func makeVoteSet(numValidators int, votingPower uint64) (*VoteSet, *ValidatorSet, []*PrivAccount) {
+func makeVoteSet(height uint32, round uint16, numValidators int, votingPower uint64) (*VoteSet, *ValidatorSet, []*PrivAccount) {
 	vals := make([]*Validator, numValidators)
 	privAccounts := make([]*PrivAccount, numValidators)
 	for i := 0; i < numValidators; i++ {
@@ -25,26 +26,177 @@ func makeVoteSet(numValidators int, votingPower uint64) (*VoteSet, *ValidatorSet
 		privAccounts[i] = privAccount
 	}
 	valSet := NewValidatorSet(vals)
-	return NewVoteSet(0, 0, VoteTypeBare, valSet), valSet, privAccounts
+	return NewVoteSet(height, round, VoteTypeBare, valSet), valSet, privAccounts
 }
 
 func TestAddVote(t *testing.T) {
-	voteSet, valSet, privAccounts := makeVoteSet(10, 1)
-	vote := &Vote{Height: 0, Round: 0, Type: VoteTypeBare, BlockHash: nil}
+	voteSet, _, privAccounts := makeVoteSet(0, 0, 10, 1)
 
-	t.Logf(">> %v", voteSet)
-	t.Logf(">> %v", valSet)
-	t.Logf(">> %v", privAccounts)
+	// t.Logf(">> %v", voteSet)
 
+	if voteSet.GetVote(0) != nil {
+		t.Errorf("Expected GetVote(0) to be nil")
+	}
+	if voteSet.BitArray().GetIndex(0) {
+		t.Errorf("Expected BitArray.GetIndex(0) to be false")
+	}
+	hash, commitTime, ok := voteSet.TwoThirdsMajority()
+	if hash != nil || !commitTime.IsZero() || ok {
+		t.Errorf("There should be no 2/3 majority")
+	}
+
+	vote := &Vote{Height: 0, Round: 0, Type: VoteTypeBare, BlockHash: nil}
 	privAccounts[0].Sign(vote)
 	voteSet.Add(vote)
 
-	t.Logf(">> %v", voteSet)
-	t.Logf(">> %v", valSet)
-	t.Logf(">> %v", privAccounts)
-
+	if voteSet.GetVote(0) == nil {
+		t.Errorf("Expected GetVote(0) to be present")
+	}
+	if !voteSet.BitArray().GetIndex(0) {
+		t.Errorf("Expected BitArray.GetIndex(0) to be true")
+	}
+	hash, commitTime, ok = voteSet.TwoThirdsMajority()
+	if hash != nil || !commitTime.IsZero() || ok {
+		t.Errorf("There should be no 2/3 majority")
+	}
 }
 
 func Test2_3Majority(t *testing.T) {
-	// XXX
+	voteSet, _, privAccounts := makeVoteSet(0, 0, 10, 1)
+
+	// 6 out of 10 voted for nil.
+	vote := &Vote{Height: 0, Round: 0, Type: VoteTypeBare, BlockHash: nil}
+	for i := 0; i < 6; i++ {
+		privAccounts[i].Sign(vote)
+		voteSet.Add(vote)
+	}
+	hash, commitTime, ok := voteSet.TwoThirdsMajority()
+	if hash != nil || !commitTime.IsZero() || ok {
+		t.Errorf("There should be no 2/3 majority")
+	}
+
+	// 7th validator voted for some blockhash
+	vote.BlockHash = CRandBytes(32)
+	privAccounts[6].Sign(vote)
+	voteSet.Add(vote)
+	hash, commitTime, ok = voteSet.TwoThirdsMajority()
+	if hash != nil || !commitTime.IsZero() || ok {
+		t.Errorf("There should be no 2/3 majority")
+	}
+
+	// 8th validator voted for nil.
+	vote.BlockHash = nil
+	privAccounts[7].Sign(vote)
+	voteSet.Add(vote)
+	hash, commitTime, ok = voteSet.TwoThirdsMajority()
+	if hash != nil || commitTime.IsZero() || !ok {
+		t.Errorf("There should be 2/3 majority for nil")
+	}
+
+}
+
+func TestBadVotes(t *testing.T) {
+	voteSet, _, privAccounts := makeVoteSet(1, 0, 10, 1)
+
+	// val0 votes for nil.
+	vote := &Vote{Height: 1, Round: 0, Type: VoteTypeBare, BlockHash: nil}
+	privAccounts[0].Sign(vote)
+	added, err := voteSet.Add(vote)
+	if !added || err != nil {
+		t.Errorf("Expected Add(vote) to succeed")
+	}
+
+	// val0 votes again for some block.
+	vote = &Vote{Height: 1, Round: 0, Type: VoteTypeBare, BlockHash: CRandBytes(32)}
+	privAccounts[0].Sign(vote)
+	added, err = voteSet.Add(vote)
+	if added || err == nil {
+		t.Errorf("Expected Add(vote) to fail, dupeout.")
+	}
+
+	// val1 votes on another height
+	vote = &Vote{Height: 0, Round: 0, Type: VoteTypeBare, BlockHash: nil}
+	privAccounts[1].Sign(vote)
+	added, err = voteSet.Add(vote)
+	if added {
+		t.Errorf("Expected Add(vote) to fail, wrong height")
+	}
+
+	// val2 votes on another round
+	vote = &Vote{Height: 1, Round: 1, Type: VoteTypeBare, BlockHash: nil}
+	privAccounts[2].Sign(vote)
+	added, err = voteSet.Add(vote)
+	if added {
+		t.Errorf("Expected Add(vote) to fail, wrong round")
+	}
+
+	// val3 votes of another type.
+	vote = &Vote{Height: 1, Round: 0, Type: VoteTypePrecommit, BlockHash: nil}
+	privAccounts[3].Sign(vote)
+	added, err = voteSet.Add(vote)
+	if added {
+		t.Errorf("Expected Add(vote) to fail, wrong type")
+	}
+}
+
+func TestAddCommitsToBareVotes(t *testing.T) {
+	voteSet, _, privAccounts := makeVoteSet(1, 5, 10, 1)
+
+	// val0, val1, val2, val3, val4, val5 vote for nil.
+	vote := &Vote{Height: 1, Round: 5, Type: VoteTypeBare, BlockHash: nil}
+	for i := 0; i < 6; i++ {
+		privAccounts[i].Sign(vote)
+		voteSet.Add(vote)
+	}
+	hash, commitTime, ok := voteSet.TwoThirdsMajority()
+	if hash != nil || !commitTime.IsZero() || ok {
+		t.Errorf("There should be no 2/3 majority")
+	}
+
+	// Attempt to add a commit from val6 at a previous height
+	vote = &Vote{Height: 0, Round: 5, Type: VoteTypeCommit, BlockHash: nil}
+	privAccounts[6].Sign(vote)
+	added, _ := voteSet.Add(vote)
+	if added {
+		t.Errorf("Expected Add(vote) to fail, wrong height.")
+	}
+
+	// Attempt to add a commit from val6 at a later round
+	vote = &Vote{Height: 1, Round: 6, Type: VoteTypeCommit, BlockHash: nil}
+	privAccounts[6].Sign(vote)
+	added, _ = voteSet.Add(vote)
+	if added {
+		t.Errorf("Expected Add(vote) to fail, cannot add future round vote.")
+	}
+
+	// Attempt to add a commit from val6 for currrent height/round.
+	vote = &Vote{Height: 1, Round: 5, Type: VoteTypeCommit, BlockHash: nil}
+	privAccounts[6].Sign(vote)
+	added, err := voteSet.Add(vote)
+	if added || err == nil {
+		t.Errorf("Expected Add(vote) to fail, only prior round commits can be added.")
+	}
+
+	// Add commit from val6 at a previous round
+	vote = &Vote{Height: 1, Round: 4, Type: VoteTypeCommit, BlockHash: nil}
+	privAccounts[6].Sign(vote)
+	added, err = voteSet.Add(vote)
+	if !added || err != nil {
+		t.Errorf("Expected Add(vote) to succeed, commit for prior rounds are relevant.")
+	}
+
+	// Also add commit from val7 for previous round.
+	vote = &Vote{Height: 1, Round: 3, Type: VoteTypeCommit, BlockHash: nil}
+	privAccounts[7].Sign(vote)
+	added, err = voteSet.Add(vote)
+	if !added || err != nil {
+		t.Errorf("Expected Add(vote) to succeed. err: %v", err)
+	}
+
+	// We should have 2/3 majority
+	hash, commitTime, ok = voteSet.TwoThirdsMajority()
+	if hash != nil || commitTime.IsZero() || !ok {
+		t.Errorf("There should be 2/3 majority for nil")
+	}
+
 }