Ethylene oxide, C2H4O, is a three-membered ring with a single oxygen atom bridging the two carbons. Reactions of H and D atoms with ethylene oxide have been studied in the gas phase to provide insight into the dynamics of three-membered ring opening. H atoms were produced by photolyzing HI in the wavelength range 240-266 nm. The channel leading to OH+C2H4 was monitored via laser-induced fluorescence (LIF) of the OH A (2) Sigma<--X (II)-I-2 system. The D atom reaction yields OD with no hydrogen scrambling. With an available energy of 23 000 cm(-1), the average OH D rotational energy is similar to 350 cm(-1) for OH(v=0) and similar to 250 cm(-1) for OD(v=1). OH(v=1) was not observed, while the OD(v=1) population was about one-tenth that of OD(v=0). Ther was no apparent bias in populations between Lambda doublets in each of the spin-orbit states for both OH and OD. Doppler broadening of OH(v=0) rotational lines was measured to evaluate the average center-of-mass (c.m.) translational energy, which was found to be similar to 2300 cm(-1). On average, the ring opening process deposits similar to 10% of the available energy into c.m. translation, similar to 2% into OH rotation, and similar to 88% into ethylene internal energy. Comparison with CH2CH2OH unimolecular dissociation dynamics and theoretical transition state calculations leads to a likely mechanism in which hydrogen abstracts oxygen via sequential C-O bond fission without involving a lone-lived CH2CH2OH intermediate.