Acidity differences of substituted acetic acids X-CH2-COOH are interpreted in terms of the nature of the group X. Traditional quantum-chemical and density functional theory based descriptors of chemical reactivity are calculated for 15 acids and are correlated with experimental acidities in aqueous solution and in the gas phase. The results are interpreted within the framework of Pearson’s hard and soft acids and bases principle, stating that hard acids prefer to-bond to hard bases and soft acids prefer to bond to soft bases. Calculated parameters, which are a measure of local hardness, show a positive correlation with the experimental acidities. The acidities in aqueous solution show almost no dependence on the softness (polarizability) of the group X, while this group softness seems to play an important role-in the gas phase, although it is a minor one in comparison to the hard parameters.