The effect of different working variables on the chlorination of MoO3 in the presence of carbon was studied under atmospheric pressure in an isothermal fixed bed reactor at laboratory scale, with continuous gas flow. The reagents used were chlorine gas and powder molybdenum trioxide and carbon black. The effects on the reaction rate of several variables-carbon content in the mixture, mixing time, flow rate, chlorine molar fraction, reaction time and temperature-were studied. The experimental results showed that (a) MoO3 conversion is markedly affected by temperature and reaction time, and slightly affected by chlorine molar fraction and flow rate; (b) the following carbochlorination reaction: 2MoO(3(s)) + C-(s) + 2Cl(2(g)) --> 2MoO(2)Cl(2(g)) + CO2(g) can be proposed from the identification of all the products obtained; (c) there is an active chlorinating species that would generate on the carbon surface. On the basis of these results, a mechanism is proposed for this complex heterogeneous solid-gas reaction that would explain the higher reactivity observed in the carbochlorination reactions at low temperatures with respect to the direct chlorination verified at temperatures higher than 500 degreesC. To justify this mechanism, a thermodynamic analysis that supports the experimental results was carried out. (C) 2002 Elsevier Science Ltd. All rights reserved.