back of catalytic converter 12 in this case output signal 31 of rear exhaust gas sensor 14 is approximately 0 volts the concentration of fuel in the fuel air mixture is increased at approximately time t 579 sec and internal combustion engine 1 is operated with a rich mixture lambda value 30 in front of catalytic converter 12 decreases to a value of approximately 0 75 output signal 31 of rear exhaust gas sensor 14 initially remains at a low value the oxygen concentration in the exhaust gas behind catalytic converter 12 only decreases after the nitrogen oxides stored in nitrogen oxide storage catalyst 12 u2032 are reduced to nitrogen i e when the regeneration phase of nitrogen oxide storage catalyst 12 u2032 has ended this results in a clearly visible increase in output signal 31 of exhaust gas sensor 14 from approximately 0 volts to an constant level of approximately 0 7 volts the regeneration phase of nitrogen oxide storage catalyst 12 u2032 is designated in fig2 by reference numeral 32 according to the present invention one may infer that regeneration phase 32 of nitrogen oxide storage catalyst 12 u2032 has ended when the gradient of output signal 31 of exhaust gas sensor 14 exceeds a specifiable limiting value i e when the slope of output signal 31 has reached a particular value alternatively one may deduce that regeneration phase 32 has ended when the gradient of output signal 31 of exhaust gas sensor 14 initially exceeds a specifiable first limiting value and then falls below a specifiable second limiting value i e the.