The lowest singlet and triplet potential energy surfaces of formaldehyde carbonyl oxide (1) and acetaldehyde carbonyl oxide (2) have been investigated in the regions concerning the most relevant unimolecular reactions by means of CASSCF and MRDCI ab initio quantum-chemical calculations. The questions related to the mechanism of O-atom loss from carbonyl oxides, as well as the competition between the cyclization to dioxirane and the tautomerization to hydroperoxide in methyl-substituted carbonyl oxides are addressed in this investigation. The theoretical predictions are consistent with experimental findings obtained from stopped-flow studies of the gas-phase ozonation of both trans-butene and tetramethylethylene. An unexpected result is that the most reasonable pathway for O-atom loss from "hot" singlet carbonyl oxides 1 and 2 involves internal rotation about the CO bond axis, followed by intersystem crossing to the lowest triplet state and subsequent scission of the OO bond.