As a first phase in the investigation of the feasibility of storing light water reactor spent fuel in air, oxidation tests were performed on nonirradiated UO2 pellets over the temperature range of 150 to 345°C. The objective of the tests was to determine the important independent variables that affect the oxidation behavior of fuel. Pellets tested at the high end of the temperature range (>230°C) oxidized very rapidly from the standpoint of projected storage periods in air. These results suggest that acceptable spent-fuel storage temperatures should be <230°C. The tests also revealed that the oxidation was initially retarded by the presence of a coating, probably a higher oxide, that formed on pellets during the period of air storage before they were tested. The oxide coating became increasingly semiprotective after longer storage periods. Other variables identified as important to oxidation behavior of fuel were temperature, radiolysis of a static air atmosphere, fuel microstructure, gadolinia content, and humidity.