The vinoxy radical, a common intermediate in gas-phase alkene ozonolysis, reacts with 02 to form a chemically activated alpha-oxoperoxy species. We report CBS-QB3 energetics for O-2 addition to the parent ((CH2CHO)-C-center dot, 1a), 1-methylvinoxy ((CH2COCH3)-C-center dot, 1b), and 2-methylvinoxy ((CH3CHCHO)-C-center dot, 1c) radicals. CBS-QB3 predictions for peroxy radical formation agree with experimental data, while the G2 method systematically overestimates peroxy radical stability. RRKM/master equation simulations based on CBS-QB3 data are used to estimate the competition between prompt isomerization and thermalization for the peroxy radicals derived from 1a, 1b, and 1c. The lowest energy isomerization pathway for radicals 4a and 4c (derived from 1a and 1c, respectively) is a 1,4-shift of the acyl hydrogen requiring 19-20 kcal/mol. The resulting hydroperoxyacyl radical decomposes quantitatively to form (OH)-O-center dot. The lowest energy isomerization pathway for radical 4b (derived from 1b) is a 1,5-shift of a methyl hydrogen requiring 26 kcal/mol. About 25% of 4a, but only similar to 5% of 4c, isomerizes promptly at 1 atm pressure. Isomerization of 4b is negligible at all pressures studied.