The hypothesis that predissociation as evidenced by sudden broadening of the rotation lines in an absorption spectrum may be used to establish the strength of the bond involved in the associated photochemical act is re-examined. It is shown to lead to incorrect conclusions in the few cases where its use seemed applicable. New values are derived for the C–C and C–H bond strengths in various compounds. The values are consistent among themselves and with observations in reaction kinetics but show a small spread due to secondary effects of adjacent double bonds. In free acetyl and formyl radicals the bonds are much weaker than in the stable compounds.