Fourier transform infrared spectroscopy has been used to investigate the bonding and chemistry of methyl groups, from the dissociative adsorption of methyl iodide, adsorbed on silica-supported copper catalysts. After reaction of methyl iodide on Cu/SiO2 at room temperature, the infrared data show the presence of at least two different types of adsorbed methyl groups'that differ in their thermal stability. The data suggest that methyl iodide initially dissociates on the Cu surface to form methyl groups and iodine atoms. Some of the methyl groups remain adsorbed on the copper particles whereas others migrate, or spillover, on to the nearby support where they react with SiOH to form SiOCH3. This is the first study to provide spectroscopic evidence for alkyl group spillover on an oxide-supported metal catalyst. Spillover of alkyl fragments has important implications for hydrocarbon catalysis.