It is proposed that the pyrolysis of ethylene oxide (EO) proceeds via an internal isomerization (similar to that in isoelectronic cyclopropane) to form an excited (CH3CHO)* molecule. It is estimated that the excess energy of this latter is about 85 kcal, which is about 7 kcal in excess of the C-C bond dissociation energy in CH3CHO. The lifetime of (CH3CHO)* for bond fission is estimated to be 10—8.5 sec so that its fate is predominantly quenching at P≥200 mm Hg. Estimates of the isomerization rate give, in good agreement with the inhibited reaction: ki=1014.1510-57/θsec-1 where θ=4.575T kcal/mole. Analysis of the chain mechanism indicates a chain length between 1 and 2 with the production of a ``hot'' CH2CHO radical formed by exothermic ring opening of the EO radical. This is probably quenched but may decompose spontaneously to CH2CO+H. Published data on the high temperature reactions (1100°K) give absolute rates in reasonable accord with the 700°K reaction. The lower values of Eact found for the higher temperatures may not be reliable. More meagre data previously published on the pyrolysis of propylene oxide are in good agreement with expectations drawn from the EO system both qualitatively and quantitatively. Other ``hot'' molecule reactions are discussed briefly.