The limits of flammability of ethylene in oxygen, air and air-nitrogen mixtures were measured at elevated temperatures and pressures in a three litre spherical bomb. The limits in oxygen were determined at 80°C, 120°C and 150°C and over the range 135 to 515 lb/in 2 abs, pressure. The limits in air and air-nitrogen mixtures were determined at atmospheric pressure, 45 and 135 lb/in 2 abs, at 20°C and 250°C. The upper limit in oxygen was raised by increasing either temperature or pressure, but the effect of pressure was small above 300 lb/in 2 abs. The upper limit in air was also raised by increasing either temperature or pressure and the critical oxygen concentration was reduced under the same conditions. Explosions in ethylene-oxygen mixtures close to the upper limit of flammability at high pressures gave abnormally high explosion pressures. This effect was attributed to the releases of energy associated with the decomposition of ethylene and the formation of significant quantities of methane.