The invention relates to a fluorination process for producing fluorinated compounds.The process consists in reacting a compound (I) corresponding to the formula ##STR00001## with an ionic fluoride of a monovalent cation. M represents H, an alkali metal, a quaternary phosphonium group or a quaternary ammonium group. Y represents SO.sub.2 and m is 1, or else Y is PO and m is 2. Z represents CR.sup.2, N or P. R.sup.1 represents an electron-withdrawing group which has a Hammet .sigma..sub.P parameter of greater than 0.4. R.sup.2 represents a carbonaceous and/or electron-withdrawing group. X represents a halogen other than a fluorine.The fluorinated compounds obtained are of use in particular as electrolytes in lithium batteries.