AbstractDetailed combustion studies in high‐pressure bombs show that the measured maximum gas pressures are always lower than theoretically calculated values and that these differences increase more than proportionally with rising loading densities. By means of experiments and calculations we show that a considerable part of the pressure difference is brought about by the expansion of the combustion chamber during pressure load.The expansion of the combustion chamber is responsible for pressure measurements that are about 5% too low at currently used pressures of 500 MPa. Furthermore the covolume yields an error of about 12% if the constants of the Abel state equation are determined using the standardized method for loading densities of 0.20 g/cm3and 0.22 g/cm3. The error of the force constant remains under 1%.As the values determined from pressure bomb measurements are to be introduced into further calculations, they must be corrected according to the combustion chamber expansion, or new measuring methods will be necessar