A method for monitoring a reducing agent solution composition in an exhaust gas system of an internal combustion engine with metered injection of the reducing agent solution upstream of an SCR catalytic converter comprises metering injection of the reducing agent solution into the exhaust gas stream, determining a measured variable of a change in water content in the exhaust gas stream in response to the injected reducing agent solution, and determining at least one indicator value of a composition of the reducing agent solution at least based on the determined measured variable. In this way, deviations from the standard composition may be taken into account during metering or be displayed as an error.