Experiments have been made with the ignition of natural gas in a simulated pipeline rupture, to study the pressure wave generated. Various conditions were used to produce the gas-air mixture, which was ignited by firing an incandescent bullet into it. The pressure waves that resulted from the ignition of the mixtures were recorded on a storage oscilloscope and/or an ultraviolet recorder via mocrophones placed at various points. The shape of the pressure wave was not always the same, due to the ignition starting at different points in the mixture. Some of the experiments were recorded on film by means of a high-speed camera. From this film we learned that the ignition of a part of the mixture is responsible for the resulting pressure wave. This part of the mixture expands at a speed varying between 50 and 70 m/s, while the flame front velocity does not exceed 15m/s. A spherically radiating monopole model, with data abstracted from the high-speed records, gives a value for the peak pressure of the wave which is comparable to the peak overpressure measured. The maximum peak overpressure recorded at a distance of 50 m from the ignition centre did not exceed 1.5 mbar. Extrapolation of the data toward shorte distances from the mixture shows that a peak-to-peak pressure wave of 10 mbar is to be expected at a distance of 10 m from the source.