The catalytic decomposition of vaporized dimethyl methylphosphonate(DMMP) by alumina-sup-ported precious metal catalysts was studied with a fixed bed flow reactor. The platinum catalyst showed the best performance for the reaction. DMMP vapor in air stream was decomposed perfectly at the beginning but the performances of the catalysts were reduced rapidly with the lapse of time. So we tried to elucidate the decomposition mechanism of DMMP and the deactivation mechanism of the catalysts in process of reaction by analyzing the reaction products and the surfaces of the catalysts. According to the test results, methoxy groups of DMMP were separated by hydrolysis reaction over the alumina support, and the methanol formed from the methoxy groups was decomposed to carbon dioxide and water by oxidation reaction over the precious metals. The decomposed chemicals containing phosphorous were mainly nonvolatile monomethyl methylphosphonate(MMMP) and methyphosphonic acid(MPA). A part of the nonvolatile products adhered strongly to the surfaces of the catalysts, so the catalysts were deactivated seriously.