Abstract: The invention relates to an arrangement for determining the oxygen buffer capacity in a catalytic converter (1) of an exhaust system. The arrangement includes an oxygen sensor (3, 32, 33) located downstream of at least one converter matrix (30, 31), where the sensor (3, 32, 33) supplies an input signal to a fuel injection control unit (4). The arrangement includes an analyzer circuit (13) for determining time information (TP) about the fluctuation of said input signal (25, 26), and calculating means for calculating the oxygen buffer capacity based on said time information (TP) when the downstream sensor is used to provide a primary control input signal (26). The invention also relates to a method of determining the buffer capacity.