Alloying of a binary system with an additional element often leads to the formation of new phases. In this work, a γ-TiAl based alloy with 3 at.% molybdenum was investigated, which was quenched from 1450 °C. Upon reheating, the formation of an orthorhombic phase was observed with the help of in situ high-energy X-ray diffraction. This phase formed at 600 °C and vanished again at 720 °C, and acts as a transition phase between the α ₂ and the γ phase. Such a transition phase has not been observed before in this type of alloy. Additionally, transmission electron microscopy was used to study the microstructure of selected sample states on a sub-micrometer level. The orthorhombic phase formed fine lamellae inside the α ₂ phase and the α ₂ ′ martensite.