Dechlorination of hexachlorobenzene (HCB) was achieved by a mixture of commercial CaO and alpha-Fe2O3 (CaO/alpha-Fe2O3) in closed systems at temperatures of 300 degrees C and 350 degrees C, which exhibited a synergic effect compared to CaO or alpha-Fe2O3 alone, and the dechlorination efficiency was dramatically enhanced. When CaO and alpha-Fe2O3 coexisted, HCB was dechlorinated by about 98% after 0.5 h reaction at 300 degrees C. All TeCB-, TrCB-, and DCB-isomers were detected, showing the presence of more than one dechlorination pathway. The pathway to form 1,2,4-TrCB must be a major pathway because the greatest amount of 1,2,4-TrCB was detected. There existed discrepancy of the material balance between the starting and dechlorinated materials. It implies that besides dechlorination other decomposition processes may be present. The mechanism of synergic effect of calcium and iron oxides was investigated.