The adsorption and degradation of the nerve agent simulant dimethyl methylphosphonate (DMMP) over UV-irradiated TiO2 powders and thin films has been investigated. Adsorption of vapor-phase DMMP on TiO2 powder is characterized by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Photochemically assisted oxidation of adsorbed DMMP is carried out in situ by irradiation of samples in the DRIFTS accessory, giving kinetic data and information on specific site binding of DMMP and catalyst poisoning. Gas-phase intermediates from a static vapor phase reaction are identified by gas chromatography-mass spectrometry analysis, and surface-bound intermediates and products are analyzed by high-performance liquid chromatography-mass spectrometry, and ion chromatography of both aqueous and organic extractions from the TiO2. Adsorbed DMMP is photodegraded in a stepwise fashion to give methylphosphonic acid, PO43-, H2O, and CO2 as products. A proposed reaction pathway is consistent with a rapid degradation of DMMP but with extensive poisoning of the catalyst by surface-bound phosphonate products.