The catalytic gas phase reaction between CF3-CCl=CH2 with HF yielding CF3-CF=CH2 was investigated regarding the reaction pathways involved. Different from the reaction under liquid phase (homogenous) conditions, in heterogeneous phase CF3-CCl=CH2 does not add HF forming the saturated intermediate CF3-CFCl-CH3 but undergoes direct Cl/F exchange at the starting olefin molecule resulting directly in the formation of CF3-CF=CH2. This might be caused mainly by the fact that under heterogeneous gas phase conditions higher temperatures are needed as compared to liquid phase conditions thus suppressing the formation of kinetically less stable CF3-CFCl-CH3 under these conditions. Since for all the reaction steps involved in this complex reaction system the Gibbs free reaction enthalpy values are close to zero [1], the selectivity towards several reaction products is strongly dependent on the reaction parameters, e.g. temperature, pressure and molar ratio of the reactants. Based on the catalytic data obtained for fluorinated chromia as catalyst, a reaction path distinguishing direct chlorine against fluorine exchange versus hydrofluorination/dehydrochlorination is proposed. (C) 2014 Elsevier B.V. All rights reserved.