The use of thermochemical cycles makes it possible to determine substituent effects on the homolytic bond dissociation free energies, BDFE, of the oxygen-acetyl bonds in substituted phenyl acetates and the nitrogen-acetyl bonds in substituted acetanilides. A linear correlation between BDFEs for the phenyl acetates and the BDFEs of the oxygen-hydrogen bonds in correspondingly substituted phenols is found with a slope equal to 0.78. An equivalent linear correlation does not exist between the BDFEs of the nitrogen-acetyl bonds in acetanilides and the nitrogen-hydrogen bonds in anilines. This difference in the two correlations of substituent effects on the BDFEs can be explained by the different degrees of stabilization of the parent molecules by the substituents on the phenyl rings. The acetanilides are in this respect extraordinary, since stabilization involving π-resonance plays an important role.