In this study, a novel two-stage process, i.e. first stage for hydrogen species generation at 150 degrees C and second stage for PCB dechlorination at 330 degrees C, was developed for efficient PCB degradation in subcritical water with nanoscale Ni/Fe particles. The obtained results showed that over 92% of PCBs was completely converted into 1-alkyl-benzenes during this process. The addition of hydrogen donors could greatly enhance PCB dechlorination in descending order: oxalic acid approximate to HAc > NaH2PO4 approximate to KH2PO4. The anion introduced along with H+ played the crucial role in overall dechlorination process due to the undesired reaction between anion and reactive nanoscale Ni/Fe. Oxalic acid was quite effective for the generation of hydrogen species by supply more available H+ without undesired side reaction. In contrast, phosphate could react with Ni/Fe rapidly to form complex precipitates on particle surface and thereby block the reactive sites of particle following suspension of dechlorination. In addition, low 150 degrees C at first stage as compared to 330 degrees C at second stage favored the dissolution and adsorption of hydrogen on Ni surface for catalyzing to form more reactive hydrogen species used for rapid dechlorination. (C) 2012 Elsevier B.V. All rights reserved.