The kinetics of the formation reaction of GaN from Ga-solutions under ammonia atmosphere was investigated by a thermogravimetric technique. The weight signal was recorded continuously as a function of time during the reaction. The weight signal can be converted into a conversion ratio, expressing the fraction of gallium that was converted into GaN during the reaction. The curves of the conversion ratio vs. reaction time have a sigmoidal shape with two pronounced parts: an induction period followed by a section with constant slope. The induction time is an important parameter related to the nucleation regime and can be used to describe the nucleation rate. The influence of experimental conditions like temperature, ammonia partial pressure and solvent concentrations, etc., on the induction time was studied. The results can be used for an optimization of the experimental conditions for a solution growth process for GaN crystal growth. (c) 2007 Elsevier B.V. All rights reserved.