diff --git a/src/main/java/com/metamx/emitter/core/Batch.java b/src/main/java/com/metamx/emitter/core/Batch.java
index b9d2360..84bc850 100644
--- a/src/main/java/com/metamx/emitter/core/Batch.java
+++ b/src/main/java/com/metamx/emitter/core/Batch.java
@@ -21,6 +21,8 @@
 
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.locks.AbstractQueuedLongSynchronizer;
+import java.util.concurrent.locks.ReadWriteLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
 
 /**
  * Buffer for batched data + synchronization state.
@@ -73,6 +75,11 @@ private static boolean isEmittingAllowed(long state)
   private static final long UNLOCK_AND_SEAL_TAG = 1;
   private static final long SEAL_TAG = 2;
 
+  /**
+   * Lock for guarding the firstEventTimestamp.
+   */
+  private final ReadWriteLock firstEventLock = new ReentrantReadWriteLock();
+
   /**
    * The emitter this batch belongs to.
    */
@@ -145,6 +152,7 @@ private boolean tryAddFirstEvent(byte[] event)
       return false;
     }
     try {
+      firstEventLock.writeLock().lock();
       int bufferOffset = emitter.batchingStrategy.writeBatchStart(buffer);
       writeEvent(event, bufferOffset);
       eventCount.incrementAndGet();
@@ -152,6 +160,7 @@ private boolean tryAddFirstEvent(byte[] event)
       return true;
     }
     finally {
+      firstEventLock.writeLock().unlock();
       unlock();
     }
   }
@@ -212,8 +221,11 @@ private void unlockAndSealIfNeeded()
     if (eventCount.incrementAndGet() >= emitter.config.getFlushCount()) {
       unlockAndSeal();
     } else {
-      long timeSinceFirstEvent = System.currentTimeMillis() - firstEventTimestamp;
-      if (firstEventTimestamp > 0 && timeSinceFirstEvent > emitter.config.getFlushMillis()) {
+      firstEventLock.readLock().lock();
+      long firstEventTimestampValue = firstEventTimestamp;
+      firstEventLock.readLock().unlock();
+      long timeSinceFirstEvent = System.currentTimeMillis() - firstEventTimestampValue;
+      if (firstEventTimestampValue > 0 && timeSinceFirstEvent > emitter.config.getFlushMillis()) {
         unlockAndSeal();
       } else {
         unlock();
@@ -222,8 +234,11 @@ private void unlockAndSealIfNeeded()
   }
 
   void sealIfFlushNeeded() {
-    long timeSinceFirstEvent = System.currentTimeMillis() - firstEventTimestamp;
-    if (firstEventTimestamp > 0 && timeSinceFirstEvent > emitter.config.getFlushMillis()) {
+    firstEventLock.readLock().lock();
+    long firstEventTimestampValue = firstEventTimestamp;
+    firstEventLock.readLock().unlock();
+    long timeSinceFirstEvent = System.currentTimeMillis() - firstEventTimestampValue;
+    if (firstEventTimestampValue > 0 && timeSinceFirstEvent > emitter.config.getFlushMillis()) {
       seal();
     }
   }
@@ -275,9 +290,12 @@ protected boolean tryReleaseShared(long tag)
         if (compareAndSetState(state, newState)) {
           // Ensures only one thread calls emitter.onSealExclusive() for each batch.
           if (!isSealed(state)) {
+            firstEventLock.readLock().lock();
+            long firstEventTimestampValue = firstEventTimestamp;
+            firstEventLock.readLock().unlock();
             emitter.onSealExclusive(
                 this,
-                firstEventTimestamp > 0 ? System.currentTimeMillis() - firstEventTimestamp : -1
+                firstEventTimestampValue > 0 ? System.currentTimeMillis() - firstEventTimestampValue : -1
             );
           }
           return isEmittingAllowed(newState);