The rates and amounts of oxygen pickup by samples of powdered zirconium dioxide have been measured as a function of temperature , oxygen pressure (10−5 to 760 mm Hg), and particle size. Many experiments, performed to correlate oxygen pickup with the mass of oxide and with the oxide surface, indicate that oxygen pickup is primarily a surface phenomenon. After complete reduction of the adsorbed surface oxygen, approximately one atomic monolayer of oxygen will adsorb irreversibly, and about 5% of the total adsorbed oxygen can be reversibly adsorbed and desorbed as molecules. The apparent activation energy for the adsorption of molecular oxygen is about 35 kcal/mole and for desorption about 53 kcal/mole, as compared to about 58 kcal/mole for adsorption of atomic oxygen. From theoretical calculations it was concluded that electron transfer must occur (e.g., oxygen adsorbs as ), and this molecular oxygen anion adsorbs as a mobile layer.