Multiple samples of moderate-burnup pressurized water reactor (PWR) and boiling water reactor (BWR) spent fuel are oxidized in controlled atmosphere dry baths at 175, 130, and 110° C. The atmosphere consists of air humidified to a dew point of either—55 or +80°C. These tests are run for up to 18.8 kh. The weight gain data from these tests are analyzed using a mathematical model incorporating the assumption that oxidation occurs by rapid grain-boundary oxygen diffusion, followed by oxidation of the individual fuel grains. The BWR fuel appears to oxidize slightly faster than PWR fuel, and the data suggest a possible increase in oxidation rate in moist air. Reasonable agreement is observed between the oxidation rate constants obtained in these long-term tests and prior short-term thermogravimetric analysis oxidation tests.