The measurement of burning velocity is important in providing results for testing the various theories of flame propagation1–4. Linnett and Wheatley5 have recently reviewed the earlier work on the effect of pressure on burning velocity, and from this and from their own experiments on ethylene–air mixtures conclude "that burning velocities are affected by pressure and that, for some hydrocarbons, as the pressure is lowered the burning velocity increases". However, some of the earlier results quoted have been obtained by methods which suffer from several defects, as pointed out by Sherratt and Linnett6; and the results of Garside, Forsyth and Townend7 on ethylene–air mixtures, also quoted by Linnett and Wheatley, show no progressive change in burning velocity with decrease in pressure. On the other hand, recent work by Badin, Stuart and Pease8 on flames of nitrogen–oxygen–butadiene 1,3 and helium–oxygen–butadiene 1,3 at atmospheric and reduced pressures has shown an increase in burning velocity with reduction in pressure to 300 mm. of mercury; a decrease for the helium mixture was observed as the pressure was further reduced.