To test proposals for the origin of oxygen absorption bands in the visible reflectance spectrum of Ganymede, the reflectance of condensed films of pure oxygen (O-2) and O-2-water mixtures and the evolution of O-2 from the films as a function of temperature were determined, Absorption band shapes and positions for oxygen at 26 kelvin were similar to those reported for Ganymede, whereas those for the mixtures were slightly shifted. The band intensity dropped by more than two orders of magnitude when the ice mixture was warmed to 100 keivin, although about 20 percent of the O-2 remained trapped in the ice, which suggested that at these temperatures O-2 molecules dissolve in the ice rather than aggregate in clusters or bubbles. The experiments suggest that the absorption bands in Ganymede’s spectrum were not produced in the relatively warm surface of the satellite but in a much colder source. Solid O-2 may exist in a cold subsurface layer or in an atmospheric haze.