Abstract: A process is described for the determination of the aging condition of an exhaust-gas catalyst for an internal combustion engine which is fitted with .lambda.-controlled regulation of the fuel/air ratio. In the process, the switching of the fuel/air ratio from rich to lean and vice versa is delayed and the content of the combustibles in the exhaust gas downstream of the catalyst is measured. With delayed switching, complete conversion of the exhaust gas only occurs when the catalyst still possesses its full oxygen-storage capability. Since the oxygen-storage capability decreases with increasing age of the catalyst, the degree of conversion or the proportion of combustible components arising in the exhaust gas when conversion is no longer complete is a direct measure of the aging condition of the catalyst. A catalyst can thus be replaced in good time before it is completely destroyed and before undesired components of the exhaust gas reach the atmosphere.