Co-grinding of monochlorobiphenyl (BP-Cl) with rare earth oxides has been conducted and the mechanochemical decomposition of BP-Cl on the oxides has been investigated by a suite of analytical methods including X-ray diffraction (XRD), thermogravimetry (TG), Raman spectroscopy, electron spin resonance (ESR), gas chromatography/mass spectrometry (GC/MS). The experimental results have been compared with the use of CaO as additive and the differences have been discussed. It has been found that the use of rare earth oxides allows an efficient decomposition of BP-Cl and the time required to complete the decomposition reaction is much shorter than that by the use of CaO.