Transmission infrared spectroscopy has been used to investigate the chemistry of methyl halides adsorbed on silica-supported copper catalysts. The infrared data show that after dissociative adsorption of CH3I on the copper particles at 298 K, methyl groups are present on the copper surface and the silica support. Methyl groups adsorbed on the copper particles are characterized by an infrared absorption band near 2920 cm(-1). Other bands in spectrum are characteristic of SiOCH3 groups. The IR data suggest that methyl groups migrate or "spillover" onto the silica support where they react with surface hydroxyl groups to form SiOCH3. Spillover of methyl groups is observed for the dissociative adsorption of CH3Br and CH3Cl on Cu/SiO2 as well, although higher temperatures are required to activate the C-Br and C-CI bonds. Possible mechanisms for methyl spillover are proposed.