The invention relates to a method for the diagnosis of a catalyst (115) arranged in the exhaust gas region (113) of an internal combustion engine (110), said method involving the determination of the oxygen storage capacity (O2_Sim) of the catalyst (115), which is performed on the basis of a diagnosis mixture lambda nominal signal course (lam_Diag_Sol) with rich and lean lambda values, pre-determined according to a measured downstream lambda signal (lam_nK_Mes). The invention also relates to a device for carrying out the method. In the event of a variation between rich/lean and lean/rich lambda values, a ramp-type modification (203) of the diagnosis mixture lambda nominal signal course (lam_Diag_Sol) is pre-determined, which begins with a rich/lean starting lambda value (201, 204), passes through the lambda value = 1, and ends with a lean/rich end lambda value (201, 204). The inventive method enables the determination of the oxygen storage capacity (O2_Sim) of the catalyst (115) with a high level of precision, providing a measure for the quality of the catalyst (115).