High-energy synchrotron X-ray diffraction is a novel and powerful tool for bulk studies of materials and is applied to the investigation of a high Nb-bearing γ-TiAl-based alloy. The results determined from ex situ diffraction patterns of differently heat-treated samples are compared with those from metallographic examinations. The diffraction angles and also the morphology of reflections on the Debye-Scherrer rings are evaluated in order to determine lattice parameters and grain sizes as well as crystallographic correlation. An in situ heating cycle from room temperature to 1362 °C has been conducted starting from massively transformed γ-TiAl. With increasing temperature, the occurrence of strain relaxation, chemical and phase separation, domain orientations, phase transitions, recrystallization processes, and subsequent grain growth can be observed. Finally, a fully lamellar structure, well oriented with respect to the high-temperature phase, is formed. The data obtained by high-energy synchrotron X-ray diffraction, which contain extremely rich information, are interpreted step by step and relate well with the ex situ observations and metallographic examinations. © 2006 Acta Materialia Inc.