A method for operating an internal combustion engine, in whose exhaust gas system an exhaust gas treatment device is positioned, a reagent, which is possibly mixed with compressed air, being introduced into the exhaust gas system upstream from the exhaust gas treatment device, and a device for performing the method. The reagent is brought to a predefined reagent setpoint pressure using a reagent pump and subsequently metered using a reagent dosing valve. The reagent pressure existing upstream from the reagent dosing valve is detected. In the framework of a diagnosis, a time curve of the reagent setpoint pressure is predefined. The reagent pressure detected during the diagnosis is compared to at least one predefined threshold value. If the threshold value is exceeded, an error signal is provided. The corresponding diagnosis of a compressed air path may be provided alternatively or additionally to the diagnosis of the reagent path.