The acid-base properties of 4-benzyl-5-(pyridin-2-yl)-4H-1,2,4-triazole-5(4H)-thione, (4-benzyl-5-(pyridin3-yl)-4H-1,2,4-triazole-5(4H)-thione, and 4-benzyl-5-(pyridin-4-yl)-4H-1,2,4-triazole-5(4H)-thione in ethanol/ water mixtures have been studied at 25 degrees C and for an ionic strength of 0.1 M by a multiwavelength spectrophotometric method. For evaluation of the pH-absorbance data, a resolution method based on the hard modeling was used. The acidity constants of each dissociation equilibria were estimated by fitting the whole spectral collected data to an established factor analysis model. The EQUISPEC program was used for determination of acidity constants. The corresponding pK(a) values in ethanol-water mixtures were determined. There is a linear relationship between acidity constants and the mass fraction of ethanol in the solvent mixtures. The effect of solvent on acid-base behavior was discussed.