The adsorption, thermal reactions, and photochemistry of methyl iodide were studied on powdered TiO2 by transmission infrared spectroscopy, and on a TiO2(110) single-crystal surface by temperature-programmed desorption (TPD) and X-ray photoemission spectroscopy (XPS). CH3I is either adsorbed molecularly or adsorbed dissociatively, forming methoxy groups on powdered TiO2 by thermal activation as observed by IR spectroscopy. The dissociation of CH3I is enhanced by the presence of surface hydroxyl groups. CH4(g) evolves thermally as a major product when CH3I reacts with powdered TiO2 in the absence of oxygen. However, only oxygen-containing reaction intermediates and products are found when oxygen is present, including CH2O(g), (CH3)(2)O-(g), CO2(g), CO2(g), and H2O(g). (CH3)(2)O-(g) is formed via a coupling reaction of two adsorbed methoxy groups. For the thermal reactions on TiO2(110), desorption of (CH3)(2)O-(g) is observed at multilayer coverages by TPD study. The formation of (CH3)(2)O-(g) On TiO2(110) occurs at much lower temperatures. It starts to evolve at similar to 200 K and reaches its maximum rate at 250-330 K depending on surface heterogeneity in the TPD study. The Williamson synthesis process is proposed for the generation of (CH3)(2)O-(g) in this case. In addition, surface iodine formed by X-ray irradiation on CH3I-covered TiO2 is not stable and is desorbed by 570 K. In the photochemistry of CH3I over powdered TiO2, when O-2 is absent, negligible reactions occur. In the presence of O-2, some CH3I is photooxidizcd to surface formate groups and CO2(g). This reaction is most likely initiated by superoxide, O-2(-), formed upon irradiation.