Several isomers with molecular formula C3H3O may exhibit unusual properties by virtue of a configurational isomerization pathway that changes the localization of an unpaired electron. A series of ab initio calculations was undertaken at the UHF, B3LYP, MCSCF, and QCISD levels with the 6-311G(d,p) basis set to ascertain the geometries, relative energies, isomerization barriers : and relevant spectroscopic properties of the lowest energy structural and configurational C3H3O isomers. The most stable structures, with QCISD relative energies (kJ mol(-1)) of the stable configurations given in parentheses, are H2C2HCO (0, 8, 8), H3C3O (47), H2C3HO (87, 100), and HC2HCHO (94, 95, 101, 107). The H2C2HCO and H2C3HO structures are found to have distinct isomers connected by relocalization pathways. The H3C3O structure possesses two favorable canonical geometries, but the single optimized structure is an average of these. Potential surfaces have been calculated along two vibrational bending coordinates for H2C2HCO and H2C3HO and along a single bending coordinate for H3C3O Extremely large amplitude bending motions are predicted for these three lowest energy structures, and at typical combustion temperatures it is likely that these molecules exist effectively as admixtures of two or more canonical structures.