The invention deals with a method to determine the fuel composition of a fuel mixture from a first fuel and a second fuel for the operation of an internal combustion engine, wherein the first and the second fuel have different octane numbers and wherein the internal combustion engine has at least one knock sensor and a closed-loop knock control. Provision is made according to the invention for the composition of the fuel mixture to be determined by means of an output signal of the knock sensor. Provision is made in many cases for the knock sensor to already be in the internal combustion engine, so that it does not present any additional expenditure of money or effort. Because the octane number of the fuel mixture is determined by the mixing ratio of the fuels involved, which have different octane numbers, in retrospect the composition of the fuel mixture can be suggested from the signal of the knock sensor under suitable operating conditions.