Absolute cross sections for single electron capture from He atoms by ground state and long-lived excited (metastable) ions C-q+ and O-q+ (q = 1, 2; impact energy less than or equal to 6 keV) have been measured by combining the techniques of translational energy spectroscopy and state-selective ion beam attenuation. Primary ions have been produced by electron impact on molecular gases in an ion source with controlled electron impact energy. C2+ and O2+ metastable ion beam fractions have been determined by means of state-selective ion beam attenuation due to single electron capture in various neutral gases, and C+ and O+ metastable ion beam fractions, by means of translational energy spectroscopy of primary ion excitation/de-excitation in collisions with He. Absolute cross sections for single electron capture from He have been determined with ion beams containing either exclusively ground state ions or defined admixtures of metastable ions, The results are compared with other available measurements and recent quantum-mechanical calculations. Some remaining discrepancies are probably due to imperfect determination of the respective metastable ion beam fractions, which may be improved by translational energy spectroscopy involving considerably higher ion energy resolution.