This work was inspired by findings of a certain, rather unusual appearance of precipitations, which geometrically occurred in an L-shape, hence the name “L phase”, within the microstructure of forging parts and plates made from the conventional (α+β) alloy Ti-6Al-4V. The parts and plates had a β annealing in common and the influence of oxygen was suspected in being responsible for the development of this phase. As each sample was exposed to an air environment, cooling inclusive. The assumption was that the material could form a sort of α case from within, by the oxygen content of the material. However, as this type of formation and its characteristic shape was never documented before, further investigations were to be aimed at. The L phase should be reproduced and characterized due to its appearances and mechanical properties. Furthermore, an attempt in annihilating the L phase should be made, in order to control the development of this phase. Therefore, a comprehensive heat treatment study was made, with a subsequent light optical microscope (LOM) study to characterize the precipitation and growth behavior of the phase. An intensive microhardness study was additionally made, with which results a mathematical model was used to calculate the diffusion coefficient for oxygen in Ti-6Al-4V. By an electron backscatter diffraction (EBSD) measurement over a zone, which was nano indented, a correlation between the orientation of the phase and its mechanical properties was investigated. To receive proof of oxygen as a main contributor for the development of the L phase, chemical analysis using wavelength-dispersive X-ray spectroscopy (WDX), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) were made. Resulting in an extensive characterization of growth behavior of the L phase, a favorite growth condition within heat treatments was found as well as ways to complete annihilation of the phase. The hardness and Young’s modulus were determined, as being considerably high compared to the ground material. Oxygen was verified as the main stabilizer of this phase and as a stronger stabilizer as aluminum for the α phase as well. Additionally, nitrides were proven within the surface zone of the specimens.