The HCO and COH formyl radical species were studied by the coupled cluster electron correlation technique and using the augmented correlation consistent polarized core-valence basis sets of quadruple-xi quality. The complete basis set limits for the equilibrium geometry and spectroscopic parameters of HCO were estimated: R-e(CH) = 1.117 Angstrom; R-e(CO) = 1.174 Angstrom; alpha(e)(HCO) = 124.6degrees. The COH isomerization energy is equal to 176 kJ/mol. The energy of the COH --> HCO transition state is equal to 107 kJ/mol. The heat of formation was found using the basis set of cc-pcv5z quality: Delta(f)Hdegrees(298.15) = 42.3 for HCO and 218.1 kJ/mol for COH. The thermochemistry of HCO and COH was studied beyond the rigid rotor harmonic oscillator approach. The results were compared with available experimental data and results of previous calculations.