Potentiometric titration has been used to measure dissociation constants of 38 monosubstituted benzoic acids in water and in 6 organic solvents (methanol, ethanol, dimethylformamide, acetonitrile, sulfolane, acetone). The results have been analyzed and interpreted from the point of view of substituent and solvent effects on the dissociation of the title substrates. It has been proved that solvents affect not only the reaction constant but also the substituent constant of the Hammett equation.