The mechanochemical dechlorination of pentachlorophenol (PCP) was explored, using CaO and SiO_2 powder as additives. The effects of the milling time and additives on the dechlorination rate were investigated. The resulting product was characterized by X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), thermogravimetric analysis (TG) and ion chromatography (IC). It is found that grinding operation dechlorinates PCP, with the formation of a mixture of inorganic chloride and amorphous carbon. Addition of quartz to the grinding mixture facilitates dechlorination. On the basis of the experimental results, the following decomposition mechanism is proposed. Decomposition predominantly proceeds by rupture of C-Cl bond in the PCP molecule, followed by the formation of inorganic chlorides.