We present a theoretical study of the unimolecular dissociation resonances of HCO in the electronic ground state, (X) over tilde(1)A', using a new ab initio potential energy surface and a modification of the log-derivative version of the Kohn variational principle for the dynamics calculations. Altogether we have analyzed about 120 resonances up to an energy of approximate to 2 eV above the H⁺CO threshold, corresponding to the eleventh overtone in the CO stretching mode (v(2)=11). The agreement of the resonance energies and widths with recent stimulated emission pumping measurements of Tobiason et al. ［J. Chem. Phys. 103, 1448 (1995)］ is pleasing. The root-mean-square deviation from the experimental energies is only 17 cm(-1) over a range of about 20 000 cm(-1) and all trends of the resonance widths observed in the experiment are satisfactorily reproduced by the calculations. The assignment of the states is discussed in terms of the resonance wave functions. In addition, we compare the quantum mechanical state-resolved dissociation rates with the results of classical trajectory calculations and with the predictions of the statistical model. (C) 1996 American Institute of Physics.