Abstract：The thermodynamics and mechanisms of the processes of adsorption of phenol and o-cresol from aqueous solution onto activated carbon have been studied. The adsorption isotherms have been measured at different temperatures. The results show that the molecules of phenol may be adsorbed in mixed style of vertical and flat orientation on the carbon's surface in low concentration range, but in vertical possition mainly at high concentrations. However, o-cresol may be adsorbed in flat orientation at low temperatures, but in a same way as phenol at high temperatures. The thermodynamics of the systems relates closely to the adsorption mechanisms, and when vertical adsorption occurring, the entropy change will be smaller. While, when flat adsorption occurring, the entropy change will be larger. Furthermore, some results of other solid-liquid adsorption systems in the literatures have also been discussed.