Binding constants for benzoic acid, its monosubstituted derivatives, and pyridine-carboxylic acids with bovine serum albumin were evaluated spectrophotometrically by utilizing the competition with 2-(4'-hydroxyphenylazo) benzoic acid (HABA). A modified Langmuir-type equation which quantitatively represented the binding of HABA to albumin was expanded to express the competitive system in which HABA and a noncolored compound competed for the same binding sites on albumin molecules in the same manner. The values of the binding constant obtained by the use of the expanded equation were irrespective of the three levels of initial concentration of the acids. A linear relation was observed between logarithm of the binding constant and pK_a for the acids except ortho-substituted benzoic acids and pyridine-2-carboxylic acid.