Transmission infrared spectroscopy and temperature programmed desorption have been used to investigate the chemistry of CH3I adsorbed on silica-supported copper nanoparticles. The following three factors affect the chemistry of CH3I on Cu/SiO2: (i) the oxidation state of the copper, (ii) the hydroxyl group coverage on the silica support, and (iii) the surface roughness of the copper particles. These three factors can be controlled by sample preparation. On reduced-Cu/SiO2 samples, C-I bond dissociation in adsorbed methyl iodide results in the formation of adsorbed methyl groups on the copper surface. The frequency of the symmetric stretch of adsorbed methyl groups is at 2913 cm-1 for copper nanoparticles with atomically smooth surface morphology and at 2924 cm-1 for copper nanoparticles with atomically rough surface morphology. In addition to methyl groups adsorbed on the copper nanoparticles, for reduced Cu/SiO2 samples with Si-OH groups present in close proximity to the copper particles, methyl groups can spill over on to the silica support and react with OH groups to form SiOCH3. The silica hydroxyl coverage also plays a role in methyl reactions on the copper particles as hydroxyl groups provide a source of hydrogen atoms. Methane and ethane are the predominant reaction products for reduced Cu/SiO2 samples with high hydroxyl group coverage whereas methane, ethane, and ethylene form on samples with low silica hydroxyl group coverage. The copper particle morphology may also play a role in the chemistry of adsorbed methyl groups as there is some evidence for slower methyl reaction kinetics on the copper nanoparticles with rough surface morphology. C-I bond dissociation in adsorbed methyl iodide occurs on oxidized-copper nanoparticles as well. The infrared spectrum taken after adsorption of CH3I on oxidized-Cu/SiO2 is consistent with the presence of adsorbed methoxy groups and bidentate formate on the oxidized-copper particles. The chemistry of methyl groups on oxidized-copper particles is similar to that of methanol on oxidized-copper particles. Finally, the use and complexities of characterizing these samples and copper catalysts in general with CO adsorption in conjunction with infrared spectroscopy are discussed.