Abstract: A method of fluorinating a solid compound of a rare earth metal to produce a fluorinated rare earth metal compound in solid form includes reacting, in a reaction zone, a solid compound of the rare earth metal and gaseous hydrofluoric acid, thus producing the fluorinated rare earth metal compound in solid form. The reaction takes place, in the reaction zone, in the presence of exogenous water, which is water that is exogenous to water that is produced in the reaction zone as water of reaction due to the reaction of the solid compound of the rare earth metal and the hydrofluoric acid. Conditions of temperature and pressure in the reaction zone avoid condensation of the exogenous water, the water of reaction when present, and the hydrofluoric acid.