PROBLEM TO BE SOLVED: To provide a rector which reduces the amount of HF produced in reactions between elemental fluorine and an organic compound.SOLUTION: In fluorination reactions in which an organic compound to be fluorinated is contacted with elemental fluorine and HF is produced as a byproduct, the elemental fluorine is contacted with the organic compound in the presence of a fluoride-adsorbing composition so that the amount of HF or another hydrogen-containing byproduct is reduced or eliminated. Reactor embodiments for the fluorination reactions are also disclosed. The fluoride-adsorbing composition contains compounds selected from the group consisting of alkali metal halides and alkaline earth halides.