A general investigation into diffusion flames burning from a liquid surface at low pressure has been made, as this type of problem is relevant to the question of the persistence of fires in aircraft flying at high altitudes. The fuels used were isododecane and aviation kerosine. Temperatures in the carbon-free parts of the flame and in the fuel were measured in various sizes of burner. The rate of consumption of fuel was measured and the effects of air velocity around the flame and of air vitiation were noted. Evidence was found to recirculation within the vapour space of the flame. The minimum air velocities required to extinguish flames from the fuel surface at low pressures were determined in a 16 in, diameter variable density wind tunnel. The effect on the extinction velocities of the fuel temperature, the presence of flameholders, and the presence of a metal plate above the flame was investigated. It is concluded that the probability of the persistence of fire in an aircraft remains high up to 40 000 ft. is much less at 50 000 ft, and may be negligible at 60 000 ft. The probability of its persistence at the higher altitudes may be greater in a damaged aircraft.