The impact of adsorbent pore size distribution (PSD) and adsorbate critical oxidation potential (COP) on competitive adsorption on activated carbon were investigated in this study. Three activated carbons with different pore size distribution were used as adsorbents-granular activated carbon (GAC) F400, and two activated carbon fibers (ACFs), namely ACC-10 and ACC-15. Binary adsorptions of 2-methylphenol/2-chlorophenol and 2-methylphenol/2-nitrophenol were conducted on all three adsorbents. Ternary adsorption of 2-methyl phenol/2-nitrophenol/2-chlorophenol was conducted on ACC-15 and F400. 2-Nitrophenol was more physically adsorbable than the other two adsorbates, while 2-methylphenol was the easiest to get oligomerized due to its low COP. ACC-10, which has the smallest critical pore size and narrow PSD, was shown to be effective in hampering oligomerization of phenolic compounds in all binary adsorption systems. ACC-15, which has larger critical pore size than ACC-10 and also narrow PSD, showed somewhat effectiveness in hampering oligomerization in binary adsorption systems and total effectiveness in ternary adsorptions. On the other hand, the wide pore size variations of F400 played a role in the oligomerization of the phenolic compounds. The Ideal Adsorbed Solution Theory (IAST) well predicted the multicomponent adsorption systems that did not undergo oligomerization. (C) 2008 Elsevier B.V. All rights reserved.