Six stable isomers and two transition structures were characterized on the C3H,0+' potential energy surface using the G2 procedure. Heat capacity corrections were made to allow the direct calculation of heats of formation at 1298 K. The most stable isomer is the methylketene radical cation (1, AH, 298 = 797.0 k J mol-I), followed by a distonic ion CH,CH,E=O (Z), only 46.5 k J mol-' higher in energy (AH, 2 9 8 = 843.5 k J mol-'). Ionized cyclopropanone (7) represents a transition structure for exchange of terminal methylene groups in the distonic ion with a comesponding barrier of 57.6 k J mol-'. There is a much larger barrier (106.7 kJ mol-') for the 1,2-hydrogen migration in the distonic ion (2) to form the more stable ionized methylketene (l), while the fragmentation of 2 to C,H;* + C O is highly endothermic (by 113.1 k J mol-I). The predicted ease of formation of the distonic ion on ionization of neutral cyclopropanone (7n) and the high barrier for further reaction would suggest that ionized cyclopropanone should show reactivity characteristic of a radical as observed for ionized cyclobutanone. Three additional C,H,O+' isomers, hydroxyallene radical cation (3), acrolein radical cation (4) and oxetene radical cation (9, lie within 15 k J mol-' of one another in energy, while a cyclic form (6) of the distonic ion is much less stable. G2 heats of formation and adiabatic ionization energies were also calculated for four of the neutral C3H,0 analogs. While the ionization energies are in good agreement with available experimental values, as are the heats of formation for acrolein (4n) and cyclopropanone (7n), the G2 heat of formation of methylketene (-65.1 f 10 k J mol-I) is almost 40 k J mol-' higher than the 'standard' value, which it is considered should be revised. Calculations with the GZ(MP2) method, which is less computationally intensive than G2, yield heats of formation for the neutral and ionic species within 2.4 k J mol-' of the G2 values.