The adsorption of organic acids on a commercial weakly basic resin, DIAION WA30, which has a single type of fixed functional group, appeared feasible technically. Three different organic acids, acetic acid (R’-COOH), malic acid (R "-(COOH)(2)) and citric acid (R"’-(COOH)(3)), were used in this experimental study. The adsorption isotherms depended on the way in which the organic acids dissociated in the solution, but were independent of the initial concentration of organic acids. The theoretical equations for the adsorption isotherms were derived by applying the mass action law. In an acetic acid with one carboxylic group in each molecule, the adsorption isotherm was expressed by a Langmuir-type equation. In a melic acid with two carboxylic groups, the theoretical equation for the adsorption isotherm was derived by considering that R "-COOHCOO- and R "-(COO-)(2) ions reacted with the amino group of the weakly basic resin. When the concentration of the acid in the solution is low (pH>4), R "-(COO-)(2) ion mainly contributes to adsorption, and as the concentration is high, the contribution of R "-COOHCOO- becomes significant. In a citric acid with three carboxylic groups, the theoretical equation was derived by considering that R"’-(COOH)(2)COO-, R"’-COOH(COO-)(2) and R"’-(COO-)(3) ions reacted with the resin. R"’-(COO-)(3) ion mainly contributes to adsorption at low concentration (pH>5), and the contribution of R"’-COOH(COO-)(2) and R"’-(COOH)(2)COO- ions become significant with increasing concentration.