The acid dissociation constants of series of substances with differing structural features in several acetonitrile-water mixtures were subjected to factor analysis (FA). A further target factor analysis (TFA) indicated that the Kamlet and Taft general equation was reduced in these mixtures to two or three terms: the independent term and that related to the polarity/polarizability π ∗ solvatochromic parameter, and additionally, to the hydrogen bond acceptor basicity β solvatochromic parameter. Thus, relationships between p K values and π ∗ or π ∗ and β values showing excellent correlations were established, which allowed us to calculate the p K values of substances in any acetonitrile-water mixture up to 70% (w/w). Also, the variation in the p K value with the percentage of acetonitrile in the hydro-organic mixtures could be explained in terms of the preferential solvation by water of electrolytes in the mixture studied.