The dissociation pK(a) values of benzoic acid and substitued benzoic acids have been determinated in water and acetonitrile-water media (10, 20 and 30 %) by potentiometric measurements. The results obtained shows that the solvents affect the acid equilibrium. The potentiometric data of benzoic compounds at various pH values (pH 2.0-11.0) were recorded. The calibration of the electrode system was done potentiometrically by Gran's method. Data were processed using the Hyperquad 2006 which calculate stability constants of the pure species by multi linear regression. The obtained results are in good concordance with literature values.