Abstract Under certain conditions, liquid fuels such as kerosine may form foams flammable at temperatures below the flash point of the fuel. Thomas found that at one atmosphere pressure kerosine foam would not burn in air but would burn in oxygen-enriched air. Since it appeared possible to produce flammable foams with certain combustible liquids in air, an investigation was undertaken to determine the conditions under which these foams would be formed. It was found that flammable foams could be produced quite readily with kerosine and other light oils in air and in nitrogen- and carbon dioxide-enriched air. Because these foams constitute a potential gas explosion hazard, a series of experiments was conducted in various enclosures to determine the maximum pressure developed when these foams were ignited. Tests were conducted in containers ranging from ca. 0·1 to 10 3 ft 3. The maximum pressure was found to be a function of the amount of foam, free space above the liquid surface and number of points of ignition. Maximum pressures of 35 lb/in 2 gauge were encountered in small containers. The rates of pressure rise were considerably smaller than those encountered with explosions through combustible vapour-air mixtures. Photographs of the flame propagating through kerosine foam in air show that the foam collapses and forms droplets as the flame approaches. The vapour around these droplets is then ignited and in turn acts as an ignition source to continue the propagation.