In this work the kinetics of the chlorination of molybdenum trioxide has been studied by thermogravimetry between 798 and 873 K. The starting temperature for the reaction of MoO3 with chlorine is determined at about 770 K. The influence of gaseous flow rate, sample mass, temperature, and chlorine partial pressure in the reaction rate is analyzed for two MoO3 samples having different particle size and morphology. The experimental conditions for chemical control of the reaction rate were established for both types of samples. An average activation energy of 211 kJ mol(-1) and a reaction order of 1 with respect to chlorine partial pressure were determined for the chlorination of MoO3 with gaseous chlorine. A complete rate equation was formulated that describes the reaction evolution of each type of solid. (C) 2012 Elsevier B.V. All rights reserved.