Potentiometric titration of pure gentisic acid, in aqueous solution, indicates a pK1, corresponding to ionization of the carboxyl group, of 3.1; the pK2, corresponding to ionization of a phenolic group, is 10.2. With molybdophosphotungstate T. S., gentisic acid produces a green color even in acid solution, permitting quantitative determination of the acid in the presence of salicylic acid, which requires alkalinization for color development. In strongly acid solutions, in which gentisic acid is present predominantly in molecular form, maximum absorbancy was observed at 330 mμ. In solutions having a pH between 5.0 and 8.0, containing predominantly gentisate ion, maximum absorbancy occurred at 320 mμ. In alkaline solutions the absorbancy changed too rapidly to permit precise measurement.