For monitoring a combustion process in an internal combustion engine wherein a fuel-air mixture is ignited with a high-frequency plasma it is provided to determine the impedance of the plasma. A high-frequency signal is applied to the resonator with a capacity that is so low that no arcing-over forms on the electrodes and the high-frequency current and the high-frequency voltage are measured. The impedance of the ignited mixture is determined from the high-frequency current and the high-frequency voltage.