In a method and a device for identifying the fuel quality in a fuel tank of an internal combustion engine, a reference volume with fuel is provided, which does not intermix with the fuel in the fuel tank when the fuel tank is refilled. After startup of the internal combustion engine, fuel from the fuel tank and from the reference volume is sequentially sucked into the internal combustion engine and parameters which depend from the fuel quality of the sucked-in fuel are measured and evaluated for both the fuel tank and the reference volume. If the parameter differ from one another, it can be concluded that the fuel quality of the fuel in the fuel tank before startup of the internal combustion engine has changed. A quantity (fuel factor) can then be taken into consideration in the mixture pre-control.