A conventional nonelectrolyte solution model which has led to successful predictive equations for solute solubility and infinite dilution chromatographic partition coefficients is extended to systems containing a tautomeric solute dissolved in a binary solvent mixture. The derived expression predicts that the tautomeric solute concentration in a binary solvent is a geometric average of the pure solvent ratios and permits calculation of solute-solvent association constants from variation of the stoichiometric tautomeric solute concentration ratios as a function of binary solvent composition. Experimental data for phenylazonaphthol dissolved in aqueous-ethanol and aqueous-acetone solvent mixtures is discussed in relation to the theoretical model.