Hexachlorobenzene (HCB) was thermally treated with calcium oxide (CaO) at 300-400 degrees C. Analyses of chloride ions and residual HCB confirmed that a dechlorination reaction had occurred. The dechlorination mechanism was investigated with a series of analytical methods including X-ray fluorescence (XRF), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The final products detected were CaCO3 by XRD and Raman spectroscopy, amorphous carbon by Raman spectroscopy, and CaCl2 by XPS. The newly produced species of CaCO3 and amorphous carbon were thought to be the ultimate fate of the C element of HCB. After identification of the final dechlorination products, we can conclude that the reaction of HCB with CaO at 300-400 degrees C is through a dechlorination/polymerization pathway, which is induced by electron transfer. An overall reaction formula for HCB reaction with CaO was proposed and was energetically quite favorable. The results are helpful for the further comprehension of the reaction mechanism for thermal dechlorination of PCDD/Fs in CaO rich matrices. (C) 2009 Elsevier B.V. All rights reserved.