Photolysis of iodomethane in the presence of oxygen in solid argon near 10 K has yielded several products detected by their vibrational absorption spectra. Isotopic labeling of reactants has proved methanal, water, hydrogen iodide, carbon monoxide, carbon dioxide, and hydro-peroxyl radicals to be significant products, and a further set of major absorptions is attributed to hydrogen hypoiodite HOI, hydrogen-bonded to methanal. Other minor vibrational features are discussed, and a possible reaction scheme is briefly outlined.