The oxidation properties of niobium were studied in the temperature range 450°–720°C at oxygen pressures 1–760 Torr. Particular emphasis was placed on correlating the morphology of the reaction products with the oxidation kinetics. During the initial, approximately parabolic oxidation period the scale appears compact, and at the transition from parabolic to linear kinetics the scale blisters and cracks. Scale formed during linear oxidation grows in a series of crude layers. From the relationships determined between the rate of oxidation and the thickness of the oxide layers, it is deduced that the rate of oxidation is controlled at all times by the diffusional properties of the oxide layer in contact with the metal phase. The sensitivity of the rate of oxidation to oxygen pressure is ascribed to the pressure sensitivity of the underlying parabolic rate constant of formation of the niobium pentoxide, and to a lesser extent to changes in the width of the oxide layers with the rate of their formation. The inversion in the temperature coefficient of the oxidation reaction as the temperature is lowered from 625°–600 °C is associated with the disappearance of layers of and from the reaction zone and the appearance of a metastable platelet phase.