The free radical HCCCO exhibits two distinct minima on the potential surface of the X(2)A’ ground electronic state, corresponding to two favorable; nonequivalent canonical structures : propynonyl (acetylenic) and propadienonyl (cumulenic). The geometries at these minima and the isomerization coordinate that couples them are characterized by ab initio calculations at Hartree-Fock and configuration interaction (CISD and QCISD) levels. At the QCISD/6-311G** level, the propynonyl structure has parameters r(HCa) = 1.067 Angstrom, r(CaCb) = 1.213 Angstrom, r(CaCc) = 1.435 Angstrom, r(CcO) = 1.183 Angstrom, theta(HCC) = 176.9, theta(CCC) = 168.5 degrees, and theta(CCO) = 133.3 degrees. Coupled cluster [CCSD and CCSD(T)] calculations of the relative energies were carried out at the QCISD optimized geometries. The propynonyl structure is consistently found to be the most stable, in qualitative agreement with recent experiments [J. Chem. Phys. 1994, 101, 178]. At the highest level of theory used, the propadienonyl structure is predicted to lie 12.5 kJ mol(-1) higher in stabilization energy. The (2)A" excited Renner-Teller state is found to be linear, with adiabatic and vertical stabilization energies of 25.6 and 165 kJ mol(-1), respectively, relative to the X(2)A’ equilibrium structure. Discrepancies between the experimental geometry and predicted equilibrium geometry are tentatively ascribed to vibrational averaging on the extremely anharmonic bending potential surface. Predicted harmonic vibrational frequencies, permanent dipole moments, and Fermi contact terms are reported, with comparison to experimental results when possible.