A study on the air oxidation behaviour of a commercially pure Ti6Al4V alloy between 600 and 700-degrees-C is reported, based on determination of the kinetic curves, microhardness profiles in the metal beneath the scale, and examination of morphology and composition of the scales. The oxidation kinetics shows a gradual transformation dependent on both time and temperature from a diffusive to a nearly linear rate law. It has been observed that such transformation may be associated with the acceleration of oxide scale growth. This phenomenon is accompanied by a parallel change of the oxide morphology, which is essentially manifested with the onset of a duplex-type scale starting from 650-degrees-C. Aluminium is found to pile up near the external surface of the oxide scales, although the presence of a continuous film of alumina may be excluded under any circumstances. Finally, the effect of aluminium and vanadium is beneficial in reducing appreciably the amount of oxygen dissolved in the surface metal layer, compared with unalloyed titanium.