The invention relates to a method for diagnosing the degree of ageing of a catalyst arranged in the exhaust gas channel of an internal combustion engine, wherein the oxygen storage capacity of the catalyst is determined by means of the signals of a waste gas probe arranged upstream and downstream from the catalyst according to the OSC method when a rapid change occurs in the air/fuel ratios between a rich and lean mixture composition supplied to the internal combustion engine. The aim of the invention is to provide a method for determining the degree of ageing of a catalyst arranged in the exhaust gas channel of an internal combustion engine, wherein it is possible to ascertain the degree of ageing of a catalyst in a more accurate and more reliable manner, requiring little measurement effort. According to the invention, it is possible to determine a degree of precious metal sintering of the catalyst and a degree of damage to the wash coat structure from the respective oxygen storage capacities determined from the OSC measurement methods. By combining the two values, it is possible to determine the degree of ageing of the catalyst.