Stress-induced gamma --> alpha(2), alpha(2) --> gamma and gamma --> 9R phase transformations in a hot-deformed Ti-45 at % Al-10 at% Nb alloy have been investigated using high-resolution transmission electron microscopy. The gamma --> alpha(2) phase transformation is an interface-related process. The interfacial superdislocations emitted from the misoriented semicoherent alpha(2)-gamma interface react with each other or with the moving dislocations in the gamma phase, resulting in the formation of the at phase. The nucleation of the alpha(2) --> gamma phase transformation takes place either at the alpha(2)-gamma interfaces or at the stacking faults on the basal plane of alpha(2) phase, and the growth of gamma plate is accomplished by the moving of a/6[1 0 (1) over bar 0] Shockley partials on alternate basal plane (0 0 0 1 )(alpha 2). The 9R structure was usually found to form at incoherent twin or pseudotwin boundaries. During deformation the interfacial Shockley partial dislocations of these incoherent twin and pseudotwin boundaries may glide on (1 1 1), planes into the matrix, resulting in the formation of 9R structure. The interfaces (including alpha(2)-gamma and gamma-gamma interfaces) as well as the crystallographic orientation relationship between the as-received or stress-induced alpha(2), gamma and 9R phase have been analysed. The mechanisms for the stress-induced gamma --> alpha(2), alpha(2) --> gamma and gamma --> 9R phase transformations were also discussed.