The dissociation constant of acetic acid, benzoic acid, 3-phenylpropenoic acid, 2-methylpropionic acid, and pentanoic acid were determined at 25.0 degrees C (I = 0.1 mol dm(-3) KNO3) by potentiometric pH titration in pure water and water + methanol, water + ethanol, water + dimethylformamide, water + dimethyl sulfoxide, and water + acetonitrile. pK(a) values increase with an increase in the concentration of the organic compounds. These results are discussed in terms of solvent characteristics. Solvent basicity and stabilization of the conjugate acid free base by hydrogen-bonding interactions in hydroorganic solvent media relative to pure aqueous media as well as proton-solvent interaction play an important role in the acid dissociation equilibrium.