diff --git a/core/src/main/kotlin/cohort/CohortStorage.kt b/core/src/main/kotlin/cohort/CohortStorage.kt
index 7e9b826..190c026 100644
--- a/core/src/main/kotlin/cohort/CohortStorage.kt
+++ b/core/src/main/kotlin/cohort/CohortStorage.kt
@@ -307,6 +307,8 @@ internal class RedisCohortStorage(
     }
 
     override suspend fun deleteCohort(description: CohortDescription) {
+        val blobKey = RedisKey.CohortBlob(prefix, projectId, description.id, description.lastModified)
+        redis.del(blobKey)
         cohortBlobCache.remove(description.id)
         redis.hdel(RedisKey.CohortDescriptions(prefix, projectId), description.id)
         val cohortMembersKey =
@@ -380,6 +382,7 @@ internal class RedisCohortStorage(
                                 description.id,
                                 "removed_${System.currentTimeMillis()}",
                             )
+                        val existingBlobKey = RedisKey.CohortBlob(prefix, projectId, description.id, prev.lastModified)
 
                         try {
                             // Server-side set operations - no memory transfer to client!
@@ -406,10 +409,11 @@ internal class RedisCohortStorage(
                                 }
                             }
                         } finally {
-                            // Clean up temporary keys
+                            // Clean up temporary and existing keys
                             redis.del(addedKey)
                             redis.del(removedKey)
                             redis.expire(existingCohortKey, ttl)
+                            redis.expire(existingBlobKey, ttl)
                         }
                     } else {
                         // No previous cohort: all members are additions
diff --git a/core/src/test/kotlin/cohort/CohortStorageTest.kt b/core/src/test/kotlin/cohort/CohortStorageTest.kt
index 3b473fa..a729540 100644
--- a/core/src/test/kotlin/cohort/CohortStorageTest.kt
+++ b/core/src/test/kotlin/cohort/CohortStorageTest.kt
@@ -20,6 +20,7 @@ import kotlin.test.Test
 import kotlin.test.assertEquals
 import kotlin.test.assertNotNull
 import kotlin.test.assertNull
+import kotlin.test.assertTrue
 import kotlin.time.Duration
 
 class CohortStorageTest {
@@ -173,6 +174,14 @@ class CohortStorageTest {
                 val b64 = redis.get(blobKey2)
                 assertNotNull(b64)
             }
+            // old blob and members keys should be marked for expiration on update
+            run {
+                val oldBlobKey = RedisKey.CohortBlob("amplitude ", "12345", cohort.id, 1)
+                assertTrue(redis.expirations.containsKey(oldBlobKey.value))
+                // existing cohort members key (lastModified=1) should also be set to expire
+                val oldMembersKey = RedisKey.CohortMembers("amplitude ", "12345", cohort.id, "User", 1)
+                assertTrue(redis.expirations.containsKey(oldMembersKey.value))
+            }
             // check cohort membership exists
             redis.sscan(RedisKey.UserCohortMemberships("amplitude ", "12345", "User", "1"), 1000)?.let {
                 assertEquals(setOf(cohort.id), it)
diff --git a/core/src/test/kotlin/test/InMemoryRedis.kt b/core/src/test/kotlin/test/InMemoryRedis.kt
index cbbb713..fba6685 100644
--- a/core/src/test/kotlin/test/InMemoryRedis.kt
+++ b/core/src/test/kotlin/test/InMemoryRedis.kt
@@ -8,6 +8,7 @@ internal class InMemoryRedis : Redis {
     private val kv = mutableMapOf<String, String>()
     private val sets = mutableMapOf<String, MutableSet<String>>()
     private val hashes = mutableMapOf<String, MutableMap<String, String>>()
+    internal val expirations = mutableMapOf<String, Duration>()
 
     override suspend fun get(key: RedisKey): String? {
         return kv[key.value]
@@ -129,7 +130,7 @@ internal class InMemoryRedis : Redis {
         key: RedisKey,
         ttl: Duration,
     ) {
-        // Do nothing.
+        expirations[key.value] = ttl
     }
 
     override suspend fun saddPipeline(