A method and a device for determining a composition of a fuel mixture, in particular for determining the ethanol content and/or a water component in the fuel mixture. Fuel flows sequentially through spatially separated sensor elements. The sensor elements generate time-dependent output signals, which are unambiguously dependent on the ethanol content of the fuel mixture. A difference between the time-dependent output signals is ascertained and subjected to a plausibility check. A received plausibility-checked output signal is ascertained, having at least one item of information about the direction of change in the ethanol content of the fuel mixture.