Ti-6Al-4V (Ti64) parts manufactured by Laser Powder Bed Fusion (L-PBF) exhibit a fine martensitic microstructure and significant residual stresses that are detrimental for their mechanical performance. Consequently, post-processing heat treatments are necessary to relieve these internal stresses and achieve a microstructure adequate for the mechanical requirements. Given the variations in powder compositions and processing parameters, there is a large scattering of physical properties of both as-built and heat-treated parts. In this context, this study quantifies the link between interstitial contents (specifically oxygen and nitrogen) and the transition temperatures of as-built parts, using as-atomized pre-alloyed powders for reference. The martensite decomposition temperature and beta transus temperature are assessed using high-temperature X-Ray diffraction and differential scanning calorimetry for both the powder and as-built parts. It was found that the martensite that composes the as-built microstructure can be decomposed into a stable α+β structure at temperatures exceeding 490 °C. Oxygen and nitrogen pick-up occurs during the L-PBF process, leading to an elevated beta transus temperature in as-built parts relative to the original powder. This insight provides a guide for optimizing post-processing heat treatments for Ti64 as-built components.