In a method for operating a fuel system of an internal combustion engine, the fuel system has a fuel tank and a ventilation device having at least one ventilation valve for ventilating the fuel tank in the direction of a device that generates a negative pressure, at least intermittently. A magnitude of deformation caused by an internal pressure of the fuel tank of the fuel tank is measured and, if the magnitude of deformation exceeds a threshold, the ventilation valve is closed and/or a bypass is opened.