A relationship between the energy content of stoichiometric pockets of natural gas-air mixture and the pressure rise resulting from their ignition has been derived from experiment and is reported in this paper. It is shown that the pressure rise is proportional to the energy available up to approximately 5 Btu/ft (185 kJ /m 3 ) and that the increase in pressure measured in pounds force per square inch is numerically equal to the energy release measured in British thermal units per cubic foot. Above about 5 Btu/ft energy release, the pressure rise increases more rapidly than is predicted by the simple linear relationship that holds at low energy releases. The way in which this relationship is modified when the chamber is fitted with vents or flues has been investigated. It is shown that the venting of combustion products through circular orifices can significantly lower the pressure rise produced until the vent is reduced to a critical size, at which combustion products cannot escape rapidly enough and the full explosion pressure is obtained as if the chamber were not vented. It is demonstrated that flues cannot in general be considered to be effective explosion reliefs. During burner start-up, unignited gas may be released into the combustion chamber where it mixes with the atmosphere. In practice only a small proportion of this gas, that is, the gas near the burner nozzles, is within the limits of flammability. However, it is shown that a weak mixture of gas and air having proportions below those of the lower flammability limit can contribute to the pressure rise, the effect becoming more marked as the temperature of the chamber is increased. The effect of flame speed on the relationships derived for natural gas-air mixtures was determined by repeating most of the experiments with town gas and propane. The implications of the experimental work on industrial appliance design are discussed as well as the application of the results to other enclosures that could contain flammable gases.