The invention concerns a method to determine the composition of a fuel from the components ethanol and gasoline of an internal combustion engine, wherein the fuel is supplied to the internal combustion engine by a fuel supply unit and via two fuel distributor rails; and the exhaust gas is discharged from the internal combustion engine by two exhaust gas manifolds of a first and a second exhaust gas system, which are separated from each other, wherein the first and second exhaust gas system have in each case at least one exhaust gas probe and wherein current changes in the ethanol content of the fuel are evaluated from the signal differences of lambda signals from the exhaust gas probes. With this method, a change in the ethanol content of the fuel can be detected, and an explicit condition for a change in fuel can be derived. Moreover, with the method, errors in the fuel-mixture can be distinguished from an altered fuel composition, which takes place after the tank has been filled fueling.