Abstract The acid–base properties of 1-(2-thiazolylazo)-2-naphthol (TAN) in mixtures of methanol–water at 25 °C and an ionic strength of 0.1 mol/L are studied by a multi-wavelength spectrophotometric method. The acidity constants of all related equilibria are estimated using the whole spectral fitting of the collected data to an established factor analysis model. DATAN program was used for determination of acidity constants. The corresponding p K a values in methanol–water mixtures were determined. There is a linear relationship between acidity constants and the mole fraction of methanol in the solvent mixtures.