To assess the flammability of the atmosphere in vehicle tanks, the hydrocarbon concentrations in the petrol tank of a motor car have been investigated by closely simulating the usual operating conditions. These covered in particular the ambient temperature and, thus the temperature of the petrol in the vehicle tank, but also the simulated movement of the tank and the influence of the vapour pressure of the petrol. Measurements of the concentration in the vapour space have shown that at very low temperatures in the vehicle tank due to overnight cooling, the formation of an explosive vapour/air mixture cannot be excluded; especially in the case of petrols whose vapour pressures lie at the lower limit of the new European standard (EN DIN 228), explosive atmosphere in vehicle tanks must be reckoned with. The flammability of such vapour/air mixtures has been confirmed by accompanying ignition tests.