The invention concerns a method to determine the composition of a fuel mixture from a first and at least a second fuel for the operation of an internal combustion engine with at least one combustion chamber, wherein the first and the second fuel have different boiling points and/or different enthalpies of evaporation. Provision is thereby made for the composition of the fuel mixture to be ascertained from the pressure in the combustion chamber and/or a parameter associated with the pressure and/or the time history of the pressure and/or the time history of a parameter associated with the pressure during and/or after an injection of fuel during a compression phase of the fuel-air mixture. An advantage of the method according to the invention is that when initially starting the engine after filling the tank (fueling), the fuel mixture ratio, which resulted from the filling of the tank (fueling), can already hereby be determined.