The pK(a) values of some arylpropionic and arylacetic acids were determined in acetonitrile water mixed solvents at 298.15 K. The retention factors of the conjugate acid base species are obtained by fitting the retention factors and pH data by nonlinear regression. The suggested model uses the pH value measured in the hydro-organic mixture used as mobile phase instead of the pH value in water and takes into account the effect of the activity coefficients. The effect of the acetonitrile concentration on the ionization constant was also studied by estimating the pK(a) at different acetonitrile volume fraction, ranging from 0.40 to 0.55. The plc values were correlated with the Kamlet and Taft solvatochromic parameters of acetonitrile + water mixtures to estimate aqueous pK(a).