Equilibrium experiment was conducted to investigate the factors determining the adsorption of benzoic acid (BA) and its derivatives, m- and p-hydroxy BA, onto SUS316L stainless steel particles of 8-10 mu m diameter and under 100 mesh. Adsorption isotherms of these benzoic compounds were determined in the presence of 0.05 M NaCl at pH 4 and 30 degrees C, The adsorptions of the these compounds were described well by a Langmuirian model for both adsorbents. When the maximum number of the benzoic compound adsorption sites was expressed on the basis of unit surface area (N, mol/m(2)), the N values were relatively constant, while the greatest value of the affinity (K, ml/mu mol) was obtained for p-hydroxy BA, although its value was in the same range as that of the other two adsorbates. Diffuse-reflectance Fourier transform infrared spectra of the fine adsorbent (8-10 mu m diameter) after equilibration suggest that the adsorption mainly takes place through the carboxyl group of the adsorbate-stainless steel surface interaction for all adsorbates, whereas concomitant interaction occurs in part with participation of the phenolic hydroxyl group for p-OH BA adsorbate, accounting for the difference in adsorption properties.