This study investigated the photodegraclation of pentachlorophenol (PCP) in two types of iron oxide/oxalate systems as a function of pH. Two iron oxides of hematite (alpha-Fe2O3) and goethite (alpha-FeOOH) were selected. The experiments conducted at pH values 3.5, 5.0, and 7.0 showed that PCP photodegradation proceeded rapidly at 3.5, and slowed down with the increase in pH. To account for the effect of pH on the degradation kinetics, the surface charge of iron oxides, the adsorption of PCP and oxalic acid onto the iron oxides at different pH values were researched. The presence of oxalic acid gave the iron oxides a negative charge at pH values above 3, which caused the decrease of PCP adsorption with increasing pH value. A higher PCP adsorption in low pH may contribute to PCP photodegradation. On the other hand, iron oxides adsorbed oxalic acid to form photoactive Fe(III)-oxalate complexes. Compared with alpha-FeOOH, alpha-Fe2O3 had a much stronger ability to adsorb oxalic acid and showed better photocatalytic activity. Furthermore, the detection results of H2O2 during the process of photodegraclation showed that a low pH favored the H2O2 generation. The combination of oxalic acid photodegradation and intermediates during the process of PCP photodegradation indicated that at an initial pH of 3.5, the PCP photodegradation occurred mainly via the attack by (OH)-O-center dot, and that at initial pH values 5.0 and 7.0, a direct photolysis mechanism was dominant. (C) 2011 Elsevier B.V. All rights reserved.