In this work, significant modification of the microstructure of a rolled Ti-45Al-5Nb (at.%) alloy has been achieved by a two-step heat treatment. A combination of high cooling rates for the first step of the treatments with a subsequent annealing at high temperatures, results in refined microstructures where substantial discontinuous coarsening seems to play an important role for the grain refinement. In order to fully understand the transformation mechanisms responsible for such a refinement, Electron Back Scattered Diffraction (EBSD) technique has been extensively used. In particular, a study of orientation relationships between the phases which are present in the transformed microstructures, was performed by this technique. Besides discontinuous coarsening process, the obtained results also suggest that a modified discontinuous coarsening mechanism is operative, in order to justify the formation of novel microstructures, whose development could not be explained by the classical discontinuous coarsening.