UV-irradiated methanol (CH3OH) in water ice at 3 K has been investigated with infrared spectroscopy and compared with pure methanol. The main byproducts detected are formaldehyde (H2CO), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4) and ethylene glycol (C2H4(OH)(2)). The production of H2CO, CO2, and CO is enhanced in water ice, resulting from cross reactions between the byproducts of methanol with those of water (OH and H2O2).