The decomposition of hexachlorobenzene (HCB) has been investigated over ultrafine Ca-Fe composite oxide catalyst (Ca/Fe atomic ratio was 3.4), CaO and alpha-Fe2O3 by using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Adsorption experiments on the surface of oxides monitored by in situ DRIFTS showed that partial oxidation products, i.e., phenolate and acetate species were formed on the surface of Ca-Fe composite oxide. The further studies indicated that Ca-Fe composite oxide catalyst was easier to induce the rupture of C-Cl bond and C-C bond of aromatic rings. The best catalytic activity of Ca-Fe composite oxide may be related to the acidity, which was determined by NH3-TPD. The products after reaction have been analyzed by XRD and chloride selective electrode. Ca-Fe composite oxide exhibited the highest extent of mineralization for organic chlorine among the different oxides tested. The combined results of current and previous experiments demonstrated that two competitive reactions took place during the decomposition process of HCB: (1) hydrodechlorination resulting in the formation of lower chlorinated benzenes and (2) oxidation of aromatic rings leading to the rupture of aromaticity and the formation of oxidation products. The latter is the major process in the coexisted competitive reaction. A possible decomposition pathway was discussed.