A method is provided for monitoring an SCR catalytic converter (12), in which method the NH3 storage capacity of the SCR catalytic converter (12) is monitored. The SCR catalytic converter (12) is initially filled in an overdosing phase with a superstoichiometric reducing agent dosing up to the maximum NH3 storage capacity. The catalytic converter is subsequently acted on in an underdosing phase with a reducing agent dosing which is reduced in relation to a normal dosing or is absent. The NH3 storage capacity is indirectly determined during the underdosing phase by determining at least one characteristic value which is dependent on the NOx conversion rate. According to the invention, to detect NH3 slippage (25), which indicates the transition from the overdosing phase to the underdosing phase, a variable which is characteristic of the NOx conversion is continuously measured during the overdosing phase, and in the event of a fall in the NOx conversion, it is inferred that NH3 slippage (25) is present.