Equilibrium geometries have been optimized and harmonic vibrational frequencies obtained for each of the neutral and protonated structures of HOCO employing augmented coupled cluster methods. Calculations on CO and CO2 have been performed to assist in calibrating the computed results. The proton affinity is estimated to be 130.7 kcal mol(-1) and is in good agreement with experiments. Adiabatic ionization potential for HOCO is estimated to be 184.4 kcal mol(-1) (8.01 eV) and is found to be discrepant with the measurements of Ruscic, Schwarz, and Berkowitz [J. Chem. Phys. 91, 6780 (1989)]. (C) 1997 American Institute of Physics.