The thermal behavior of two active carbons both saturated with o- and m-chlorophenols has been studied by differential thermogravimetry (DTG) and by mass spectrometric temperature-programmed desorption (TPD-MS). These two active carbons differed mainly by their surfaces and porosities. The comparison of the results of both techniques showed that the desorption may be divided into three steps, i.e., first, desorption of physisorbed chlorophenol; second, evolution of oxygen containing light gases; and third polymerization with evolution of hydrogen. Diffusional resistance did not affect our DTG or TPD-MS spectra. The amount of initially adsorbed chlorophenol, the bonding strength of physisorbed chlorophenol initially present on the surface of the carbon, and the transformation of physisorbed chlorophenol into chemisorbed species appeared to play an important role in the thermal degradation of adsorbed chlorophenols. The production of light gases depends on a particular chemical property of the surface of carbon which is likely the amount of involved oxygen groups.