Different materials have been ground in Comex jets mills and the influence of the mill operating parameters such as mill filling level, distance between nozzles, feed material and rotor speed, on the grinding capacity was investigated. Additionally, the influence of the construction parameters on the operation of the mill were also established. The Comex mill was tested to produce different size fractions from various feed materials, and it was also found, if desired, that it is capable of producing fine particles with a small size reduction ratio. Comex jet mills were installed in a plant operating under different conditions. Further, they were compared in terms of product stability and particle size. After optimisation, some jet mills have operated at a higher grinding capacity for the same feed material and final product, when compared with other units. (C) 2004 Elsevier Ltd. All rights reserved.