An offset in the signal of a pre-catalytic converter lambda probe of an exhaust gas system of an internal combustion engine affects a measured oxygen intake storage capacity and a measured oxygen removal storage capacity of an oxygen store with identical magnitude, but with opposite mathematical sign, so that their sum is independent of the offset. The oxygen intake storage capacity and the oxygen removal storage capacity are hereby determined until the output signal of the post-catalytic converter probe exceeds an intermediate threshold value of, for example, 0.45 V for intake and 0.8 V for removal. Exposure of the oxygen store to rich and/or lean exhaust gas is maintained after this threshold value has been crossed to ensure that the oxygen store is indeed sufficiently filled after the oxygen intake storage capacity has been measured, or is sufficiently emptied after the oxygen removal storage capacity has been measured.