Two samples of activated carbons were used in this study: one of coal origin and one of wood origin. The initial samples were modified by impregnation with urea followed by heat treatment at 450 and 950 degreesC. The pore structure and surface chemistry of the initial and modified samples were characterized by using sorption of nitrogen, Boehm and potentiometric titration, and thermal analysis. The results showed differences in structural characteristics and in the surface chemical inventory. Gas chromatography at infinite dilution was used to study the adsorption of acetaldehyde. The isosteric heats of adsorption were calculated from the values of the retention volumes. After urea modification, which resulted in an increase in the total number of surface groups and a decrease in the surface area and the micropore volume, the strength of the interactions of acetaldehyde with the carbon surface decreased. The adsorption forces are strong in small pores, and their volume governs the amount adsorbed. This amount can be enhanced when functional groups containing nitrogen are present. These groups can provide additional adsorption centers when all high-energy centers, small pores, are filled with acetaldehyde molecules.