New aliovalent-doped chromium oxide and fluoride catalysts have been prepared by coprecipitation techniques to obtain solid solutions. The catalysts have been characterized by, e.g., X-ray, BET, and pyridine adsorption analysis, and tested for their catalytic activity in the gas-phase reaction of C2Cl4 with HE The latter reaction involves several consecutive steps and produces the expected C2HClxFy (x + y = 5) compounds, but also some perhalogenated compounds the formation of which needs further explanation. Reaction data obtained with a (Cr,Mg)(F,OH)(x) catalyst were used to calculate reaction rates and activation energies of all the consecutive steps which could be successfully used to predict the reaction outcomes under varying experimental conditions. (C) 2003 Elsevier Inc. All rights reserved.