A number of specimens of uranium dioxide have been prepared and studied in a non-adiabatic non-isothermal calorimeter at -183 degrees C. Uptake of oxygen by the oxide is rapid. Incremental addition of oxygen has yielded differential heats of adsorption which decrease on all specimens, as the surface becomes covered. The initial heat of adsorption is 55 $\pm $ 2 kcal/mole. The rate of evolution of heat becomes increasingly slow for successive increments of oxygen adsorbed. This slow process takes place after the disappearance of all the oxygen from the gas phase and must therefore be a reaction in or on the oxide. It is discussed in terms of a slow change to the chemisorbed state of oxygen molecules rapidly adsorbed initially in the physical state.