The acid-base properties of 4-(2-pyridylazo)resoreinol in binary mixtures of acetonitrile + water are studied by a multiwavelength spectrophotometric method. To evaluate the pH-absorbance data, a resolution method based on the combination of soft- and hard-modeling is applied. The acidity constants of all related equilibria are estimated using the whole spectral fitting of the collected data to an established factor analysis model. The corresponding pK(a) values have been determined in all binary mixtures of acetonitrile + water. There is a linear relationship between acidity constants and the mole fraction of acetonitrile in the solvent mixtures. The effect of solvent properties on acid-base behavior is discussed.