The dissociation pK(a) values of benzoic acid and substituted benzoic acids have been determined in water and acetonitrile-water mixtures (10%, 20%, 30%) by spectrophotometric measurements. The results obtained show that the solvents affect the acid equilibrium. Acetonitrile acts as a strongly differentiating solvent with a modest solvating power for many polar ionic solutes. Linear relationships between pK(a) values in acetonitrile-water mixtures and pK(a) values in pure water have been established for fourteen benzoic acids. rhe pHab is a computer program using the calculation dissociation constants from spectrophotometric data obtained from solutions at known pH.