The mechanochemical dechlorination of pentachlorophenol (PCP) was studied using CaO and SiO2 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 could dechlorinate PCP, with the formation of inorganic chloride and amorphous carbon. The addition of quartz to the grinding mixture facilitated dechlorination. On the basis of the experimental results, the decomposition mechanism was proposed. Decomposition predominantly proceeds through rupture of C-Cl bond in PCP molecule, followed by the formation of inorganic chlorides. The mechanic dechlorination of pentachlorophenol (PCP) was studied using CaO and SiO2 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) spectra (FT-IR), thermogravimetric analysis (TG) and ion chromatography (IC). It is found that grinding operation could dechlorinate PCP, with the formation of inorganic chloride and amorphous carbon. The addition of quartz to the grinding mixture facilitated dechlorination. On the basis of the experimental results, the decomposition mechanism was proposed. Decomposition predominantly proceeds through rupture of C-Cl bond in PCP molecule, followed by the formation of inorganic chlorides.