Ab initio complete active space SCF calculations have been performed to determine the structures and energetics of the electrophilic addition of O(3P) to aliene (propadiene). The long-established channel for this reaction has been O-atom attack of the central carbon atom (CCA). However, recent experimental studies have suggested that terminal carbon attack (TCA) is a significant process. In this study the classical barrier height for the CCA channel was found to be only 1.8 kcal/mol lower than that for the TCA channel. In addition, the triplet diradical species and the experimentally observed product of the TCA channel, allenyloxy radical, are characterized. Allenyloxy radical is predicted to have low-lying near-IR and UV transitions to electronic excited states 0.525 and 3.70 eV above the ground state. Similar transitions for the analogous vinoxy radical have previously been predicted theoretically and confirmed experimentally. © 1990 American Chemical Society.