Single and binary solute adsorption of phenol and o-cresol at 23°C on three activated carbons with different pore size were conducted in this study. One granular activated carbon (GAC) F400 and two activated carbon fibers (ACFs) were considered. Anoxic (absence of molecular oxygen) and oxic (presence of molecular oxygen) conditions were evaluated. For the single solute system, the three adsorbents studied have shown higher adsorptive capacities under oxic conditions as compared to anoxic conditions. The degree of adsorption enhancement was related to the pore size of the adsorbent. For binary adsorption on ACC-10, which has the least average pore size (1.76 nm), the oxic and anoxic adsorption isotherms overlapped, indicating no impact of the presence of molecular oxygen on the adsorptive capacity. Significant differences on adsorptive capacities were noticed for the binary solute adsorption on ACC-15 (2.10 nm) and F400 (2.48 nm). The Ideal Adsorbed Solution Theory (IAST) predicted well all binary systems for ACC-10 and anoxic isotherms on ACC-15 and F400. Poor model predictions for GAC and ACC-15 under oxic conditions were attributed to the oligomerization of the adsorbates on the surface of activated carbon. The unique pore size of ACFs has been shown to be a limiting factor in hampering the oligomerization under oxic conditions.