diff --git a/bundles/org.openhab.core.auth.oauth2client/src/main/java/org/openhab/core/auth/oauth2client/internal/OAuthClientServiceImpl.java b/bundles/org.openhab.core.auth.oauth2client/src/main/java/org/openhab/core/auth/oauth2client/internal/OAuthClientServiceImpl.java
index 767dc56e4c..bb5d434cae 100644
--- a/bundles/org.openhab.core.auth.oauth2client/src/main/java/org/openhab/core/auth/oauth2client/internal/OAuthClientServiceImpl.java
+++ b/bundles/org.openhab.core.auth.oauth2client/src/main/java/org/openhab/core/auth/oauth2client/internal/OAuthClientServiceImpl.java
@@ -291,7 +291,7 @@ public class OAuthClientServiceImpl implements OAuthClientService {
     }
 
     @Override
-    public AccessTokenResponse refreshToken() throws OAuthException, IOException, OAuthResponseException {
+    public synchronized AccessTokenResponse refreshToken() throws OAuthException, IOException, OAuthResponseException {
         if (isClosed()) {
             throw new OAuthException(EXCEPTION_MESSAGE_CLOSED);
         }
@@ -333,7 +333,7 @@ public class OAuthClientServiceImpl implements OAuthClientService {
     }
 
     @Override
-    public @Nullable AccessTokenResponse getAccessTokenResponse()
+    public synchronized @Nullable AccessTokenResponse getAccessTokenResponse()
             throws OAuthException, IOException, OAuthResponseException {
         if (isClosed()) {
             throw new OAuthException(EXCEPTION_MESSAGE_CLOSED);