Standard pH values; pH(s), for five reference buffer solutions, KH tartrate, KH₂ citrate, KH phthalate, acetate buffer and phosphate buffer in acetonitrile–water mixtures containing 10, 30, 40, 50, 70 and 100 wt. % acetonitrile at 298.15 K have been determined using the IUPAC standardization rules. The relationship between pH(s) and solvent composition, expressed as a fraction, have been studied, with a view to assessing the presence of preferential solvation effects. In order to obtain pH(s) values for all possible acetonitrile–water mixtures, the linear solvation energy relationships method, LSER, has been applied. The pH(s) values were then correlated with the Kamlet–Taft, π*, α and β solvatochromic parameters of the acetonitrile–water mixtures. The equation obtained permits the standardization of potentiometric sensors in these mixtures.