The ordering of the bcc phase present in the Ti-Al-X (X = Ta, Nb, or Mo) system is investigated in this paper. There is a ternary phase of ordered bcc (B2) structure near the composition Ti-25Al-25X. All compositions mentioned in this paper are in atomic percent, unless mentioned otherwise. The phase reactions in Ti-33Al-17Ta alloy are examined in detail following solidification and solid-state processing treatments. It can be found from the differential thermal analysis ( DTA) that the B2 phase in this alloy is stable up to 1205degreesC, where it experiences a solid-state order to disorder transformation. The rate of this ordering reaction is so high that it cannot be arrested by rapid solidification processing (RSP). The presence of thermal antiphase boundaries (APBs) in the microstructure confirms the solid-state ordering of the B2 phase from the disordered bcc phase. Ordering reactions involving the bcc phase in the Ti-Al-Nb and Ti-Al-Mo systems are also studied because of their similarity with the Ti-Al-Ta system. It can be found that they are also solid-state ordering reactions. In the Ti-25Al-25Nb alloy the B2 phase is ordered up to 1141degreesC; whereas in the Ti-25Al-25Mo alloy the B2 phase is ordered up to 1418degreesC. (C) 2003 Kluwer Academic Publishers.