The adsorption properties of activated carbons prepared by treating a densified refuse-derived fuel by KOH activation were studied. At a low KOH/char ratio of 2, the carbons exhibited a BET surface area up to 1655 m2 g−1 and contain both micropores and mesopores. The studied carbons showed excellent adsorption capability for 4-chlorophenol, methylene blue, and the dye acid blue 74. For example, the adsorption capacities of 4-chlorophenol, methylene blue, and acid blue 74 on the carbons prepared at a KOH/char ratio of 3 were 494, 571, and 413 g kg−1, respectively. A linear fit using the Toth equation of the present data along with data for 38 other adsorption systems reported in the literature was used to characterize liquid-phase adsorption. It was found that the exponent T of the Toth equation can represent the degree of heterogeneous adsorption. The adsorption of small molar-mass solutes (e.g., 4-chlorophenol) occurs on quasi-homogenous surfaces (T ∼ 1), whereas the adsorption of larger molar-mass solutes (e.g., acid blue 74) appears to occur on heterogeneous surfaces (T < 0.34).