Molybdenum, being a strong β stabilizer, is an important alloying element in TiAl alloys, since a significant volume fraction of the disordered bcc β-phase at elevated temperatures improves the processing characteristics during hot-working. Unfortunately, the effect of Mo on the individual phases and their transition temperatures is not completely known but is necessary for designing engineering applications. In this paper, sections of the Ti-Al-Mo ternary phase diagram derived from thermodynamic calculations as well as experimental data are presented. Further, the phase transition temperatures given by the phase diagrams are compared with results from isothermal heat treatment studies, differential scanning calorimetry measurements and in-situ high-temperature diffraction experiments. Combining all of these results, a revised phase diagram is proposed.