Being a strong β stabilizer, Mo has gained importance as an alloying element for so-called β/γ-TiAl alloys. Intermetallic TiAl-based alloys which contain a significant volume fraction of the body-centered cubic β-phase at elevated temperatures have proven to exhibit good processing characteristics during hot-working. Unfortunately, the effect of Mo on the appearing phases and their temperature dependence is not well known. In this work, sections of the Ti-Al-Mo ternary phase diagram derived from thermodynamic calculations as well as experimental data are presented. The phase transition temperatures stated in these phase diagrams are compared with the results of high-temperature diffraction studies using high-energy synchrotron radiation. Additionally, the disordering temperature of the βo-phase is determined. Intermetallic TiAl alloys with high contents of disordered body-centered cubic β-phase are known to exhibit good processing characteristics during hot-working. Mo is one of the elements used for stabilizing this phase but has an otherwise not very well investigated influence on the phase equilibria. To shed light on the effect of Mo on the phase diagram, two model alloys with different Mo contents were produced and investigated by microscopic and diffraction techniques.