The reactions of iodomethane, diiodomethane, and formaldehyde over stoichiometric, O-terminated, and Cl-terminated á-Cr2O3 surfaces, were studied by thermal desorption spectroscopy. Adsorbed formaldehyde forms dioxymethylene species on the nearly-stoichiometric surface that react primarily above 600 K. Dioxymethylene decomposes via a Cannizzaro-type process with dehydrogenation to formate, and hydrogenation to methoxide. Methoxide hydrogenation produces methane and methanol near 670 K. Formate decomposition occurs at 720 K, producing acetylene, carbon monoxide, carbon dioxide and formic acid. The site requirements for these reactions are a cation/anion site pair. When the surface cations are capped with Cl, the reactivity associated with dioxymethylene intermediates above 600 K is not observed. At higher coverages, polymerization to paraformaldehyde is observed on both surfaces, and decomposition to formaldehyde is observed at 400 K in thermal desorption. Iodomethane and diiodomethane are used as sources of methyl and methylene surface species, respectively. Methyl fragments on the stoichiometric surface dehydrogenate to surface methylene and hydrogen as a rate limiting step to produce ethylene and methane at 505 K. On the oxygen-terminated surface, the methyl fragments undergo dehydrogenation and coupling to ethylene at 425 K, undergo oxygen insertion to formaldehyde at 425 K, and produce carbon dioxide, formic acid, and water above 700 K from the dehydrogenation of formate. Methylene fragments on the stoichiometric surface undergo diffusion limited coupling to ethylene at 390 to 490 K and produce methane at 520 K from dehydrogenation. On the oxygen-terminated surface, methylene undergoes oxygen insertion to produce formaldehyde at 450 K, produce carbon monoxide, formaldehyde, and water at 695 K from dioxymethylene dehydrogenation, and produce carbon dioxide, formic acid, and water above 700 K from the dehydrogenation of formate.