Adsorption of seven phenolic compounds, namely phenol, 2-methylphenol, 2-ethylphenol, 2-chlorophenol, 2-nitrophenol, 4-chlorophenol, and 4-nitrophenol on granular activated carbon (GAC) F400 and two activated carbon fibers (ACFs), ACC-10 and ACC-15 were conducted to study the impact of functional groups on oligomerization. Single solute adsorption and binary solute adsorption were conducted under anoxic (absence of molecular oxygen) and oxic (presence of molecular oxygen) conditions at 23 +/- 1 degrees C. For F400, the critical oxidation potential (COP), influenced by substitution of functional group, was found to be a limiting factor in oligomerization of adsorbates. For ACC-10, which has narrow pore size distribution (PSD) and small critical pore diameter (8.0 A), only phenol and 2-methylphenol showed occurrence of oligomerization under oxic conditions. The degree of oligomerization under oxic conditions was related to the PSD of the adsorbent. The micro-pore diameter of the ACFs effectively reduced the oligomerization process. The oxic and anoxic binary adsorption isotherms of phenol/2-ethylphenol and 2-methylphenol/2nitrophenol on ACC-10 overlapped indicating hampering of oligomerization by the limited micro-pore diameter of ACFs. The Ideal Adsorbed Solution Theory (IAST), using the Myers equation for correlating the single-solute anoxic isotherms, well predicted both anoxic and oxic binary isotherms. (c) 2007 Elsevier B.V. All rights reserved.