In biological treatment processes combined with the use of activated carbon, adsorbability and desorbability receive special attention. In particular, desorbability is of crucial importance for bioregeneration of activated carbon in these systems. This study aims to clarify the effect of the type of activated carbon on the extent of adsorption of phenol and the reversibility of adsorption. For this purpose, four different types of activated carbon: thermally-activated and chemically-activated powdered carbons (PAC), and their granular countertypes (GAC) with similar physical characteristics, were used. Adsorption and desorption isotherms showed that thermally-activated carbons, either in powdered or granular form, were better adsorbers for phenol than chemically-activated ones. However, apparently higher degrees of hysteresis (higher adsorption irreversibility) were calculated in the case of thermally-activated carbons. Chemisorption was the dominant adsorption mechanism for thermally-activated carbons, whereas physisorption was dominant for chemically-activated ones. Degrees of hysteresis for the PACs were found to be only slightly higher than their GAC countertypes, but desorption was faster from the PACs. The results suggest that, rather than the physical form, the type of carbon activation and the chemical characteristics of the surface play a more important role on the adsorbability of phenol and its reversibility. (c) 2005 Society of Chemical Industry.