Adsorption on granular activated carbon (GAC) is gaining prominence as a method for removing organic pollutants from water. The impact of the characteristics of GAC on adsorption capacity and on the potential for polymerization of phenolic compounds on the surface of GAC in the presence of molecular oxygen is evaluated in this study. Adsorption isotherm data were collected for p-chlorophenol on five activated carbons; three manufactured from bituminous coal, one manufactured from lignite coal, and one manufactured from wood. These isotherms were collected under anoxic (absence of molecular oxygen) and oxic (presence of molecular oxygen) conditions. Higher extraction efficiencies of the carbons used in the oxic isotherms were obtained for the carbons that exhibited lower increases in capacities when compared to anoxic isotherms. Furthermore, no impact of the presence of oxygen on adsorption capacity was noted for the wood base carbon. Breakthrough curves developed for p-chlorophenol on the five carbons studied have shown tailing effects for the carbons that demonstrated differences in capacities under oxic and anoxic conditions whereas no tailing was noticed for the wood base carbon.