Method for the determination of the bulk modulus of fuels in a fuel system of a combustion engine with a common rail fuel injection system, with a high pressure volume, comprising the high pressure side of a high pressure pump and a fuel accumulator with injectors for injection of fuel into cylinders of the combustion engine, a pressure sensor arranged to measure the fuel pressure in the fuel accumulator and a temperature sensor arranged to measure the temperature of the fuel in the fuel accumulator, comprising the step 2) of determining the bulk modulus of a fuel by a) supplying the fuel to the fuel system, b) controlling the high pressure pump, so that it performs a pump stroke to supply a given amount of fuel to the high pressure volume while keeping said injectors closed, c) measuring the fuel pressure in the high pressure volume before (P 1) and after (P 2) said pump stroke and as a result determining the fuel pressure increase (ΔP) achieved by the pump stroke, d) measuring the temperature of said fuel, and then calculating the bulk modulus of the fuel, based on the fuel's temperature, pressure increase (ΔP) and a value of the volume of the high pressure volume, characterised by a step 1) carried out before step 2) of 1) determining said value of the volume to be used in said calculation of the bulk modulus of a fuel by a) supplying a fuel with a known bulk modulus (B 0) to the fuel system, b) controlling the high pressure pump, so that it performs a pump stroke to supply a given amount of fuel to the high pressure volume while keeping said injectors closed, c) measuring the fuel pressure in the high pressure volume before (P 1) and after (P 2) said pump stroke, and as a result determining the fuel pressure increase (ΔP) achieved by the pump stroke, d) measuring the temperature of said fuel, and e) calculating the volume of the high pressure volume, based on the values of said temperature, fuel pressure increase (ΔP), amount of fuel supplied and the fuel's bulk modulus (B 0). a) supplying the fuel to the fuel system, b) controlling the high pressure pump, so that it performs a pump stroke to supply a given amount of fuel to the high pressure volume while keeping said injectors closed, c) measuring the fuel pressure in the high pressure volume before (P 1) and after (P 2) said pump stroke and as a result determining the fuel pressure increase (ΔP) achieved by the pump stroke, d) measuring the temperature of said fuel, and 1) determining said value of the volume to be used in said calculation of the bulk modulus of a fuel by a) supplying a fuel with a known bulk modulus (B 0) to the fuel system, b) controlling the high pressure pump, so that it performs a pump stroke to supply a given amount of fuel to the high pressure volume while keeping said injectors closed, c) measuring the fuel pressure in the high pressure volume before (P 1) and after (P 2) said pump stroke, and as a result determining the fuel pressure increase (ΔP) achieved by the pump stroke, d) measuring the temperature of said fuel, and e) calculating the volume of the high pressure volume, based on the values of said temperature, fuel pressure increase (ΔP), amount of fuel supplied and the fuel's bulk modulus (B 0). a) supplying a fuel with a known bulk modulus (B 0) to the fuel system, b) controlling the high pressure pump, so that it performs a pump stroke to supply a given amount of fuel to the high pressure volume while keeping said injectors closed, c) measuring the fuel pressure in the high pressure volume before (P 1) and after (P 2) said pump stroke, and as a result determining the fuel pressure increase (ΔP) achieved by the pump stroke, d) measuring the temperature of said fuel, and e) calculating the volume of the high pressure volume, based on the values of said temperature, fuel pressure increase (ΔP), amount of fuel supplied and the fuel's bulk modulus (B 0).