A gamma-based titanium aluminide (Ti-48Al-2Cr-2Nb-1B) has been thermal oxidation (TO) treated in an air-circulating furnace. The microstructure, composition and mechanical properties (hardness and elastic modulus) of the TO-treated material were characterised using a variety of techniques including X-ray diffraction, glow discharge spectroscopy, scanning electron microscopes, cross-sectional transmission electron microscopy, microindentation and nanoindentation tests. The load bearing capacities of the oxide layers were evaluated using both static microindentation tests and progressive scratch tests. It has been found that when TO treated at 850 degreesC for more than 8 h, a hard multi-layered oxide scale consisting of TiO2, Al2O3, Ti2AlN and TiN is formed on the titanium aluminide surface. The oxide scale is supported by a diffusion zone with a smooth hardness gradient, resulting in a high load bearing capacity. Based on the experimental results and systematic analyses, the structure of the oxide layer, the mechanical properties (hardness and elastic modulus) and the load bearing capacity of the TO-treated material are discussed. (C) 2004 Elsevier B.V. All rights reserved.