Potentiometric titrations of phenothiazines derivatives were performed in methanol-water, ethanol-water, acetonitrile-water and dioxane-water mixtures with varying contents of organic solvent. All titrations were performed in aqua-organic medium at constant ionic strength (0.15 mol.dm−3) and at different temperatures (25 to 45°C). The pKa were determined at different aqua-organic proportions. Effect of temperature and dielectric constant on dissociation constant has been compared. The pKa values were then obtained by Yasuda-Shedlovsky extrapolation.