The effect of traces of water on the relative strengths of acids (Delta pK(a) values) in acetonitrile was quantitatively evaluated experimentally and computationally (COSMO-RS). Water affects first of all the anions by selective solvation. Expectedly, the more localized is the charge in acid anions the higher is the effect of water. The energetic effect of increasing water content from 0 to ca. 10 000 ppm on solvation enthalpies of anions ranged from 0.2-0.4 kcal mol(-1) (anions with delocalized charges) to 15 kcal mol(-1) in the case of the highly charge-localized acetate ion. In the case of Delta pK(a) values the change ranges from 0.01 to ca. 1.7 pK(a) units (acid pair involving acetic acid). The COSMO-RS method was found to satisfactorily describe the trends in Delta pK(a) values. To quantify the extent of charge localization/delocalization in anions a parameter, weighted average positive sigma (WAPS), was introduced, which can be conveniently computed using the COSMO approach. WAPS characterizes the distribution of charge density across the molecular surface and was found to correlate well with the extent of water influence on the dissociation of the respective acid.