The acid dissociation constants of 3-(m-nitrophenyl)-1,5-diphenyl formazan (MNF) and 3-(p-nitrophenyl)-1,5-diphenyl formazan (PNF) have been determined in acetonitrile-water binary mixtures [50%, 60% and 70% (v/v)] by spectrophotometric method at 25 degrees C and an ionic strength of 0.1 mol .L-1 KCl. The absorption spectra were recorded in solutions of varying pH-values. Data evaluation was performed using the STAR software program which calculates stability constants and molar absorbances of the pure species by multilinear regression. A linear relationship between acidity constants and the mole fraction of acetonitrile in the solvent mixture was observed. The effect of solvent properties on acid-base behavior was discussed. The effect of nitro groups attached to para and meta positions on the ionization constant were investigated and the results were recorded.