A method for determining composition of a fuel blend from a first and at least a second fuel for operating an internal combustion engine. An ignition angle of the internal combustion engine is altered and a non-intermittent fluctuation in torque, which is a function of ignition angle, and/or a non-intermittent rotational discontinuity, and/or a variable derived from the discontinuity is used as a parameter for determining the composition of the fuel blend.