The present disclosure relates to a device and a method for monitoring an SCR exhaust gas after-treatment device. The method involves monitoring of a ratio between reducing agent quantity and nitrogen oxide conversion, especially a ratio between ammonia quantity and nitrogen oxide conversion, of the SCR exhaust gas after-treatment device. The nitrogen oxide conversion is detected or determined with a cross sensitivity to ammonia. The method furthermore involves determining of an ammonia slip condition based on the monitored ratio between reducing agent quantity and nitrogen oxide conversion. The method may offer the benefit of being easily carried out and implemented in an easy manner.