The adsorbability of 22 ortho-, meta- and para-mono- or-disubstituted benzoic acid derivatives by carbon black from aqueous solution (pH 7.4) was correlated with that on graphite from solution (pH 7.0), but not with pK_a, partition coefficient and plain size of molecule. The adsorbability of o-nitro, -methyl, -chloro and -bromobenzoic acids was less than that of the respective metha- and para-substituted compounds. The thermodynamic functions for ortho-substituted benzoic acids also were different from those for meta- and para-substituted ones, except for amino- and hydroxybenzoic acids. Plotting the solubility of caffein in the solutions containing benzoic acid derivatives and the free energy change of complex formation with caffein, respectively, against the adsorbability of the acid derivatives on carbon black, there were found correlations in both cases without excepting for ortho-substituted ones. A linear relationship was observed between the free energy change of binding to serum albumin and the adsorbability on carbon black of benzoic acid derivatives without excepting for ortho-substituted ones. These two results suggested that both the complex formation with caffein and the protein binding of benzoic acid derivatives take place on the basis of the electrostatic and the hydrophobic interactions.