A process is proposed for determining the lambda value upstream from the exhaust catalytic converter in which variation in the lambda value (Δλ) from the stoichiometric value is determined on the basis of variation in the charging (ΔOSC) of the oxygen reservoir (7) of the exhaust catalytic converter (6) and variation in the charging (ΔOSC) of the oxygen reservoir (7) is ascertained from the voltage signal (Uλdownstream) of a binary lambda probe (8) associated with the exhaust catalytic converter (6). In accordance with the proposal a control probe upstream from the exhaust catalytic converter can be omitted; this results in cost savings.