A capacitive occupant detection system (10) comprises a sine signal generator (12) to apply a sine voltage signal to an antenna electrode (14) and a current measurement circuit (18, 20, 28, 30, 40) to measure current signals, based upon which a control and evaluation unit determines and outputs an occupancy state. The signal generator (12) is coupled to the antenna electrode via an amplitude adjustment stage (13), configured to adjust the amplitude of said sine voltage signal applied to said antenna electrode to an amplitude selected among at least two discrete amplitudes. The control and evaluation circuit selects one of the discrete amplitudes at a time and causes the amplitude adjustment stage to adjust the amplitude of said the voltage signal applied to the antenna electrode accordingly. The control and evaluation circuit carries out an interference detection mode and an occupant detection mode. In interference detection mode, the current signals are measured while a low one of the at least two discrete amplitudes is selected and it is determined whether the antenna electrode is exposed to interference. In occupant detection mode, the current signals are measured while a high one of the at least two discrete amplitudes is selected and occupancy state is determined based upon the current signals thus measured.