Abstract: The time profile of the output signal of an NOx measurement transducer arranged downstream of the NOx storage catalyst is used, during and after the regeneration phase, to derive a criterion as to whether the quantity of regeneration agent to be supplied to the NOx storage catalyst in a regeneration phase must be changed in order to achieve an optimum action of the exhaust-gas purification system. The output signal is picked off at the amperometric NOx measurement transducer at two electrodes. The measurement transducer exhibits the two-position behavior necessary for the method.