Thermogravimetric analysis (TGA) and microstructural observations were carried to investigate the nitridation mechanism of beta-Ga2O3 powder to GaN under an NH3/Ar atmosphere. Non-isothermal TGA showed that nitridation of beta-Ga2O3 starts at similar to 650 degrees C, followed by decomposition of GaN at similar to 1100 degrees C. Isothermal TGA showed that nitridation follows linear kinetics in the temperature range 800-1000 degrees C. At an early stage of nitridation, small GaN particles (similar to 5 nm) are deposited on the beta-Ga2O3 crystal surface and they increase with time. We proposed a mechanism for the nitridation of Ga2O3 by NH3 whereby nitridation of beta-Ga2O3 proceeds via the intermediate vapor species Ga2O(g). (C) 2010 Elsevier B.V. All rights reserved.