The kinetics of the pyrolysis of ethylene oxide gas has been studied, by a static method, at temperatures in the range 348.4°C. to 406.7°C. and at a pressure of 15 mm. The major decomposition products, as determined by gas chromatography, were hydrogen, carbon monoxide, methane ethane, ketene and acetaldehyde. Very small amounts of methyl ketene dimer were also found. Isomerization of ethylene oxide to acetaldehyde also occurs on the reactor surface. Biradical intermediates seem to be involved both in the gas phase isomerization (in the transition state) and in the surface process (in the adsorbed state).The surface catalyzed isomerization and the observed inhibition of acetaldehyde production by the pyrolysis products are discussed theoretically. Finally activation energies and frequency factors are obtained for all four homogeneous primary processes.