Thermally treated and oxidized active carbon samples were investigated. The chemical character of active carbon surface was estimated by using water vapor adsorption, thermogravimetric measurements, and X-ray photoelectron spectroscopy. Porous structure was analyzed on the basis of benzene vapor isotherms. Oxidation as well as heating in an argon atmosphere changed markedly the concentration of the surface oxygen functional groups without substantial changes in the porous structure of the carbons. Excess adsorption isotherms from ternary solutions benzene (1) + diethyl ketone (2) + n-heptane (3) on chemically modified active carbon samples were measured. Analysis of the excess adsorption isotherms of benzene from equimolar solutions diethyl ketone + n-heptane and analysis of the changes of this binary solvent composition indicated a close correlation between the concentration of functional groups on the carbon surface and adsorption of liquid components.