The invention relates to a gas phase process for the manufacture of chloropentafluoroethane by the action of hydrofluoric acid on dichlorotetrafluoroethane in the presence of a catalyst, the said catalyst being prepared by reacting, in a gaseous phase, an alumina in which the sodium oxide content is below 300 ppm and the volume of the pores having a radius equal to 40 angstroms or above is greater than 0.7 cm.sup.3 /g, with hydrofluoric acid or with a mixture of hydrofluoric acid and of air, nitrogen or a fluorinated compound.