The effect of different organic-water mixtures on the acidity constants of alizarine red s (9,10-dihydro-3,4dihydroxy- 9,10-dioxo-2-anthracenesulfonic acid) were determined at 25 degrees C and an ionic strength of 0.1 M by a multiwavelength spectrophotometric method. Two pK(a) values for the -OH derivatives were determined. The organic solvents used were the amphiprotic (methanol, ethanol), dipolar aprotic (dimethyl sulfoxide), and low basic aprotic (acetonitrile). 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 using an established factor analysis model. The data analysis program DATAN was applied for determination of acidity constants. Generally, the pK(a) values increase with an increase in the content of the organic solvent. This behavior can be accounted for in terms of the high stabilization of both the nonprotonated and the ionic forms of this compound by dispersion forces rather than by hydrogen bonding. There are linear relationships between acidity constants and the mole fraction of different solvents in the mixtures. The effect of solvent properties on acid-base behavior is discussed.