A method for determining the composition of a fuel in a lubricant in an internal combustion engine in disclosed. The composition of a fuel having at least a first portion of a first fuel component and a second portion of a second fuel component is predefined. The mass flow with which the fuel is introduced into the lubricant in an introduction phase or discharged from the lubricant and from the housing in a discharge phase is determined. The composition of the mass flow is determined from a first mass flow of the first fuel component and a second mass flow of the second fuel component, which are determined based on a) an introduction parameter in the introduction phase or a discharge parameter in the discharge phase, and b) the first portion of the first fuel component and the second portion of the second fuel component in the fuel.