diff --git a/internal/controller/observables.go b/internal/controller/observables.go
index adbaba3f..59ceadd4 100644
--- a/internal/controller/observables.go
+++ b/internal/controller/observables.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"sync"
 
+	"github.com/aenix-io/etcd-operator/pkg/set"
 	clientv3 "go.etcd.io/etcd/client/v3"
 	appsv1 "k8s.io/api/apps/v1"
 	corev1 "k8s.io/api/core/v1"
@@ -24,6 +25,7 @@ type etcdStatus struct {
 type observables struct {
 	statefulSet    appsv1.StatefulSet
 	stsExists      bool
+	endpoints      corev1.Endpoints
 	endpointsFound bool
 	etcdStatuses   []etcdStatus
 	clusterID      uint64
@@ -43,15 +45,43 @@ func (o *observables) setClusterID() {
 
 // inSplitbrain compares clusterID field with clusterIDs in etcdStatuses.
 // If more than one unique ID is reported, cluster is in splitbrain.
+// Also if members have different opinions on the list of members, this is
+// also a splitbrain.
 func (o *observables) inSplitbrain() bool {
+	return o.clusterIDsAllEqual() && o.memberListsAllEqual()
+}
+
+func (o *observables) clusterIDsAllEqual() bool {
+	ids := set.New[uint64]()
 	for i := range o.etcdStatuses {
 		if o.etcdStatuses[i].endpointStatus != nil {
-			if o.clusterID != o.etcdStatuses[i].endpointStatus.Header.ClusterId {
-				return true
+			ids.Add(o.etcdStatuses[i].endpointStatus.Header.ClusterId)
+		}
+	}
+	return len(ids) <= 1
+}
+
+func (o *observables) memberListsAllEqual() bool {
+	type m struct {
+		Name string
+		ID   uint64
+	}
+	memberLists := make([]set.Set[m], 0, len(o.etcdStatuses))
+	for i := range o.etcdStatuses {
+		if o.etcdStatuses[i].memberList != nil {
+			memberSet := set.New[m]()
+			for _, member := range o.etcdStatuses[i].memberList.Members {
+				memberSet.Add(m{member.Name, member.ID})
 			}
+			memberLists = append(memberLists, memberSet)
+		}
+	}
+	for i := range memberLists {
+		if !memberLists[0].Equals(memberLists[i]) {
+			return false
 		}
 	}
-	return false
+	return true
 }
 
 // fill takes a single-endpoint client and populates the fields of etcdStatus
@@ -67,7 +97,11 @@ func (s *etcdStatus) fill(ctx context.Context, c *clientv3.Client) {
 	wg.Wait()
 }
 
-// TODO: make a real function
+// TODO: make a real function to determine the right number of replicas.
+// Hint: if ClientURL in the member list is absent, the member has not yet
+// started, but if the name field is populated, this is a member of the
+// initial cluster. If the name field is empty, this member has just been
+// added with etcdctl member add (or equivalent API call).
 func (o *observables) _() int {
 	if o.etcdStatuses != nil {
 		for i := range o.etcdStatuses {
diff --git a/pkg/set/set.go b/pkg/set/set.go
new file mode 100644
index 00000000..a1b5131c
--- /dev/null
+++ b/pkg/set/set.go
@@ -0,0 +1,29 @@
+package set
+
+type Set[E comparable] map[E]struct{}
+
+func New[E comparable](vals ...E) Set[E] {
+	s := Set[E]{}
+	for _, v := range vals {
+		s[v] = struct{}{}
+	}
+	return s
+}
+
+func (s Set[E]) Add(vals ...E) {
+	for _, v := range vals {
+		s[v] = struct{}{}
+	}
+}
+
+func (s Set[E]) Equals(other Set[E]) bool {
+	if len(s) != len(other) {
+		return false
+	}
+	for k := range s {
+		if _, has := other[k]; !has {
+			return false
+		}
+	}
+	return true
+}