During the last 20 years, research and development efforts have been undertaken to develop g-TiAl based alloys as a replacement for Ni-based superalloys for high-temperature applications in turbine blades of advanced aero engines and turbo-chargers of automotive combustion engines. [1] Intermetallic g-TiAl based alloys are intended for use in the temperature range from 600 to 900 8C and there is growing demand for alloys which can be deformed easily by cost-effective forming operations. One possible approach is to design alloys with microstructures in which homogenously distributed b and g-phase are the main constituents. [2] Therefore, these alloys are named b/g-alloys.[3] Since Mo is a strong b stabilizer the ternary system Ti-Al-Mo is well suited for studying this type of alloys. [4] Despite the importance of Mo as an alloying element, the knowledge of its effect on phase diagram, transformation temperatures, and order/ disorder transition temperatures is limited. In many advanced multi-phase TiAl alloys three intermetallic phases, g, a 2 , and b o , are the dominating microstructural constituents, all of which are ordered at room temperature. At elevated temperatures the ordered hexagonal a 2 -phase (D0 19 ) disorders to a (A3) and the ordered cubic b o -phase (B2) disorders to the body-centered cubic b-phase (A2) while the g-phase (L1 0 ) remains ordered up to its dissolution temperature (T g ). The phase fractions present have a strong influence on the mechanical properties of the material and on the processing characteristics at hot-working temperatures. High b/b o -phase contents, for instance, improve the deformation behavior during hot-working but simultaneously decrease the creep resistance if prevailing at service temperature. [5] Additionally, the room temperature ductility is negatively affected by high b o -phase fractions. [6] In the present work, sections of the ternary phase diagram Ti-Al-Mo, obtained by thermodynamic calculations and COMMUNICATION Being a strong b stabilizer, Mo has gained importance as an alloying element for so-called b/g-TiAl alloys. Intermetallic TiAl-based alloys which contain a significant volume fraction of the body-centered cubic b-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 b o -phase is determined. 306 wileyonlinelibrary.com ß