Heterogeneous photodegradation of pentachlorophenol (PCP) in the goethite (alpha-FeOOH) and hematite (alpha-Fe2O3) systems with oxalate under UVA illumination was investigated. The PCP degradation, dechlorination and detoxification, in terms of Microtox acute toxicity, were all achieved to the higher efficiency in the hematite suspension than in the goethite suspension. The optimal initial concentration of oxalic acid (C-0x(0)) for the PCP degradation with goethite and hematite under the experimental conditions was found to be 1.2 mM, since sufficient Fe(III) as Fe(C2O4)(3)(3-) and Fe(II) as Fe(C2O4)(2)(2-) can be formed at C-0x(0) >= 1.2 mM. The main intermediates of PCP degradation were identified by GC-MS, HPLC and IC analyses. It was found that the cycling process between Fe(III) and Fe(II) in both the goethite and hematite systems occurred more vigorously at the initial stage and gradually became gentle, while the rate of PCP photodegradation varied from fast to slow during the reaction time. Furthermore, the formation of H2O2 during photoreaction was studied to explore its relationship with the photodegradation efficiency and the iron cycling process. (C) 2009 Elsevier B.V. All rights reserved.