Binary adsorption equilibria of aqueous acetic and formic acid mixtures on activated carbon were measured. The multicomponent forms of the Langmuir, Langmuir-Freundlich isotherm, and the ideal adsorbed solution theory were used to predict the fluid phase and adsorbed phase concentration of components at equilibrium. Among the models tested, the ideal adsorbed solution theory gave uniformly satisfactory predictions both for liquid phase and for adsorbed phase compositions.