In a circuit for and a method of detecting combustion-relevant variables in an internal combustion engine an ionization signal is determined in dependence on the flame. To improve the accuracy of detection the ionisation measurement circuit is connected to the ignition electrode and decoupled from the ignition coil by way of a decoupling means. In the method a voltage potential is built up in a measurement voltage storage unit in the ignition phase with an ignition current. During at least one measurement phase the voltage potential is applied by way of a measurement resistor to an ignition electrode, wherein the ignition voltage is decoupled from the measurement voltage storage unit or from the measurement resistor when the voltage falls below a threshold voltage.