The effects brought about by several substitutions on the acidity of 16 phenol derivatives have been studied by using atoms in molecules (AIM) theory on electron charge densities obtained at the B3LYP/6-3l++G**HB3LYP/6-31G** level. The obtained results allow us to conclude that when the substituent induces an increase in the acidity of the compound. Increases are also found in the strength of the OH and CO bonds. Atomic populations and delocalization indexes can also be related to the acidic character, although these properties as calculated by AIM theory cannot be related to predictions provided by the resonance model. (C) 2004 Elsevier B.V. All rights reserved.