The reduction of WO3 with CO was studied using thermogravimetry, evolved gas analysis, X-ray powder diffraction, and scanning electron microscopy. The intermediate phases W20O58, W18O49, and WO2 were observed in the reduction. The final product of the reduction with CO was WC, compared with the tungsten formed when hydrogen and/or carbon was used. The reactant-to-product gas ratio has a considerable influence on the reactions taking place. The morphology of the sample was characterised at different stages of the reduction. The kinetics and mechanism of the reduction of WO3 with CO were studied in isothermal experiments, from 650 to 900 degrees C. Reduction occurred at a phase boundary with an activation energy of 40 kJ mol(-1). The reduction of WO2 was studied under similar conditions. The reaction also occurred at a phase boundary and had an activation energy of 62 kJ mol(-1).