The adsorption and decomposition of Dimethyl methylphosphonate (DMMP) on oxidized and reduced WO@sub 3@ surfaces have been studied with Calibrated Thermal Desorption Spectroscopy (CTDS) and X-ray Photoelectron Spectroscopy (XPS). XPS was used to characterize the surface by monitoring the valence band and W4f regions. The reduced surface, produced by Ar+ sputtering, contained a mixture of W@super 6+@ and W@super 5+@. Treatment of the surface in O2 at high temperature resulted in an oxidized surface. On the oxidized surface, desorption of methanol and dimethylether was detected, but only dimethylether was observed on the reduced surface. These products imply that the P-OCH@sub 3@ bond is broken to produce the methoxy intermediate during decomposition of DMMP on reduced and oxidized WO@sub 3@ surfaces. This mechanism is similar to the decomposition of DMMP on MgO and Al@sub 2@O@sub 3@ oxide surfaces.@footnote 1,2@ No phosphorus-containing compound was observed in CTDS. After CTDS experiments, XPS shows that phosphorus was left on the surface. The adsorption and desorption of methanol and dimethylether on the reduced and oxidized surfaces were compared with the production of these species from the DMMP decomposition. @FootnoteText@ @footnote 1@ M.K.Templeton and W.H.Weinberg, J. Am. Chem. Soc., 107 (1985) 97. @footnote 2@ Mark B.Mitchell, V.N.Sheinker and Eric A.Mintz, J. Phys. Chem. B, 101 (1997) 11192 .