Method for determining the composition of the fuel used for the normal operation of an internal combustion engine, - wherein dynamic pressure oscillations, assignable to a cylinder of the internal combustion engine, in the inlet tract of the respective internal combustion engine are measured at a defined operating point during intake-synchronous fuel injection during normal operation, and from these, a corresponding pressure oscillation signal is generated, and wherein, at the same time, a crankshaft phase angle signal is determined, and - wherein, from the pressure oscillation signal, taking into consideration the crankshaft phase angle signal detected at the same time, the actual phase position of a selected signal frequency of the measured pressure oscillations in relation to the crankshaft phase angle is determined using discrete Fourier transformation, - wherein the selected signal frequency is an intake frequency or a multiple of the intake frequency of the internal combustion engine, characterized in that, - in addition to the determined actual phase position of the selected signal frequency in the case of intake-synchronous fuel injection, a further comparison phase position is determined in the same way from the selected signal frequency in the absence of fuel injection or in the case of direct fuel injection into the closed combustion chamber, and in that - an actual phase position difference between the actual phase position and the comparison phase position is determined, wherein, - on the basis of the determined actual phase position difference, taking into consideration reference phase position differences of the same signal frequency for different fuel compositions, the fuel composition of the presently used fuel is determined. - wherein dynamic pressure oscillations, assignable to a cylinder of the internal combustion engine, in the inlet tract of the respective internal combustion engine are measured at a defined operating point during intake-synchronous fuel injection during normal operation, and from these, a corresponding pressure oscillation signal is generated, and wherein, at the same time, a crankshaft phase angle signal is determined, and - wherein, from the pressure oscillation signal, taking into consideration the crankshaft phase angle signal detected at the same time, the actual phase position of a selected signal frequency of the measured pressure oscillations in relation to the crankshaft phase angle is determined using discrete Fourier transformation, - wherein the selected signal frequency is an intake frequency or a multiple of the intake frequency of the internal combustion engine, characterized in that, - in addition to the determined actual phase position of the selected signal frequency in the case of intake-synchronous fuel injection, a further comparison phase position is determined in the same way from the selected signal frequency in the absence of fuel injection or in the case of direct fuel injection into the closed combustion chamber, and in that - an actual phase position difference between the actual phase position and the comparison phase position is determined, wherein, - on the basis of the determined actual phase position difference, taking into consideration reference phase position differences of the same signal frequency for different fuel compositions, the fuel composition of the presently used fuel is determined.