The physicochemical properties of potassium guaiacol-4-sulfonate (I) and -5-sulfonate (II) have been investigated. Analyses of the X-ray diffraction patterns, infrared spectra, and thermogravimetric and differential scanning calorimetric curves showed that I occurs as crystals that are anhydrous or hydrated up to 2/3 mole (4.8%) and that II occurs as crystals that are anhydrous or dihydrated (13.5%). Solutions of I and II at various pH values showed almost the same UV spectra at less than pH 6.0 but very different UV spectra greater than pH 7.0. Most commercially available potassium guaiacolsulfonate consists predominantly of potassium guaiacol-4-sulfonate (as determined by UV-absorption measurements and liquid chromatography).