We present measurements of the optical properties of solid O2 and O2:H2O films and on the stability of oxygen in water ice at different temperatures. The experiments are meant to test hypotheses for the location of condensed molecular oxygen in Ganymede. We find that oxygen is not retained in water ice at Ganymede's recorded surface temperatures and analyze current questions associated with the existence of oxygen in this Jovian moon. The results suggest that solid oxygen and possibly other condensed gases exist on cold patches on the heterogeneous Ganymede's surface. ¿ 1998 American Geophysical Union