Within the scope of the invention, a device for monitoring the quality of a fuel stored in a fuel tank has been developed. The main field of application is diesel-operated vehicles. The fuel is therefore preferably diesel fuel. The device is defined by the fact that means for determining the boiling point of the fuel are provided. Furthermore, a method for monitoring the quality of a fuel stored in a fuel tank has also been developed within the scope of the invention. The main field of application is diesel-operated vehicles. The fuel is therefore preferably diesel fuel. The method is defined by the fact that the boiling point of the fuel or a deviation of this boiling point from a normal value is measured. During the analysis of real injection pumps which have failed it has been detected that an excessively low boiling point of the fuel can cause the fuel to outgas. As a result, bubbles in which the necessary lubrication is no longer provided locally form in the injection pump. Furthermore, the lubricating effect is dependent on the fuel having a certain minimum viscosity. A low boiling point is also correlated with a low viscosity. Therefore, overall, the boiling point (initial boiling point, IBP) is a particularly good indicator especially of those deviations from standard values for the fuel which entail particularly costly damage.