Ti-6Al-4V alloys manufactured by laser or electron powder bed fusion (L-PBF and E-PBF) with or without hipping treatment have different microstructures from foundry alloys. Their oxidation kinetics at high temperatures between 500 and 600 °C for durations up to 2,000 h were compared. The effect of oxidation on their room temperature tensile embrittlement was quantified. It was shown that the growth kinetics of the brittle fracture zone, of the zone with cracks at 1% strain, and of the oxygen diffusion zone were perfectly correlated. Therefore, the embrittlement was confirmed to be due to oxygen ingress below the oxide scale and the kinetics were independent of the microstructure.