The unimolecular decomposition of ethylene oxide (oxirane) and the oxiranyl radial is examined by molecular orbital calculations, Rice-Ramsperger-Kassel-Marcus (RRKM)/Master Equation analysis, and detailed kinetic modeling of ethylene oxide pyrolysis in a single-pulse shock tube. It was found that the largest energy barrier to the decomposition of ethylene oxide lies in its initial isomerization to form acetaldehyde, and in agreement with previous studies, the isomerization was found to proceed through the (CH2CH2Ocenter dot)-C-center dot biradical. Because of the biradical nature of the transition states and intermediate, the energy barriers for the initial C-O rupture in ethylene oxide and the subsequent 1,2-H shift remain highly uncertain. An overall isomerization energy barrier of 59 +/- 2 kcal/mol was found to satisfactorily explain the available single pulse shock tube data. This barrier height is in line with the estimates made from an approximate spin-corrected procedure at the MP4/ 6-31 +G(d) and QCISD(T)/6-31G(d) levels of theory. The dominant channel for the unimolecular decomposition of ethylene oxide was found to form CH3 + HCO at around the ambient pressure. It accounts for > 90% of the total rate constant for T > 800 K. The high-pressure limit rate constant for the unimolecular decomposition of ethylene oxide was calculated as k(1),infinity(s(-1)) = (3.74 x 10(10))T-1.298 e(-29990/T) for 600 < T < 2000 K.