The dechlorination of pentachlorophenol (PCP) using nano-Fe together with different size of nano-Ni particles (30, 50, 80, and 100 nm) was investigated. The results indicated that nano-Ni particles could improve the dechlorination of PCP. The X-ray powder diffraction (XRD) analysis suggested that nano-Ni particles might serve as catalyst for dechlorination. The decrease of nano-Ni particle size resulted in the increase of PCP transformation and chloride release. The accumulation of several intermediates, such as phenol. 2-chlorophenol, 3-chlorophenol and 4-chlorophenol indicated the probable changes of the reaction pathway for PCP dechlorination. The corrosion products of Fe were detected using XRD analysis. In the system without nano-Ni particles, they were lepidocrocite (gamma-FeOOH) and magnetite (Fe3O4) and/or maghemite (Fe2O3), however, in the systems with nano-Ni particles, they were still magnetite/maghemite but no lepidocrocite existed. The size of nano-Ni particles might affect the crystallization of corrosion products of Fe, but did not affect the species distribution of corrosion products. (C) 2010 Elsevier B.V. All rights reserved.