diff --git a/src/coreclr/tools/r2rtest/ProcessRunner.cs b/src/coreclr/tools/r2rtest/ProcessRunner.cs
index 90792f1bc7cb2f..4afda3cae9c7a2 100644
--- a/src/coreclr/tools/r2rtest/ProcessRunner.cs
+++ b/src/coreclr/tools/r2rtest/ProcessRunner.cs
@@ -284,7 +284,10 @@ private void StandardOutputEventHandler(object sender, DataReceivedEventArgs eve
         if (!string.IsNullOrEmpty(data))
         {
             WriteLog(data);
-            _outputCapture.AppendLine("  " + data);
+            lock (_outputCapture)
+            {
+                _outputCapture.AppendLine("  " + data);
+            }
         }
     }
 
@@ -294,7 +297,10 @@ private void StandardErrorEventHandler(object sender, DataReceivedEventArgs even
         if (!string.IsNullOrEmpty(data))
         {
             WriteLog(data);
-            _outputCapture.AppendLine("!! " + data);
+            lock (_outputCapture)
+            {
+                _outputCapture.AppendLine("!! " + data);
+            }
         }
     }