TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Compound: The fluorosulfonic acid or its salt is of formula: R-S(=O)n-CF2-SO3H, R-S-CF2-SO3H, R-S(=O)n-CF2-SO2X, R-S-CF2-SO2X or R-S(=O)n-CF2-S-CH2-Ph, excluding CF3SO2CF2SO2F, FSO2CF3SO2CF2SO2F and CF3SO2CF2SO2CF2SO2F.X = halogen atom.Preparation (claimed): Sulfur-containing compounds of formulae R-SM and Y-CH2-SO3M0 are reacted to obtain fluorosulfonic acid or its salt of formula R-S-CH2-SO3H.R = (all optionally substituted) alkyl, haloalkyl, alkenyl, haloalkenyl, alkynyl, haloalkynyl, cycloalkyl, cycloalkenyl, aryl, alkoxy carbonyl, heterocyclic group, alkenyloxy carbonyl, aryloxy carbonyl, alkynyloxy carbonyl, acyl, carbamoyl or cyano group;M = H or metal atom;Y = halogen; andM0 = metal atom.Alternatively, sulfur-containing compound of formula R-S-CH2-SO3H, is fluorinated, and then formed into a salt of formula: R-S-CF2-SO3H. Sulfur-containing compound of formula: R-S-CF2-SO3H, is oxidized, and then formed into a salt of formula: R-S(=O)n-CF2-SO3H. Sulfur-containing compound of formula R-S(=O)n-CF2-SR1, is oxidized, and then formed into a salt of formula R-S(=O)n-CF2-SO3X. Sulfur-containing compound of formula R-S(=O)n-CF2-SO2X, is hydrolyzed and then formed into a salt of formula: R-S(=O)n-CF2-SO3H.R1 = (all optionally substituted) alkyl, haloalkyl, cycloalkyl or aryl group. EXAMPLE - Methanol solution (150 ml) of p-chlorobenzene thiol (in g) (26) and aqueous solution of sodium hydroxide (7.98) were mixed, and stirred at room temperature for 30 minutes. Sodium salt of chloromethane sulfonic acid (22.9) was added to the mixture, stirred at 160 degrees C for 17 hours, and concentrated under reduced pressure. Ethyl acetate was added to the residue, and crystal which was not dissolved in ethyl acetate was filtered. The crystal was washed using ethyl acetate, and dried to obtain crude crystal (40). The obtained crystal was purified using ethanol, to obtain methanesulfonic acid (4-chloro phenylthio) with a yield of 56%.    The product was fluorinated. Heptane (160 ml) and the (4-chloro phenylthio) methane sulfonic acid (20.9) were mixed and stirred at room temperature for 16 hours. Water was added to the product, and organic layer was extracted thrice using dichloromethane. Fluorobenzene (1.88 ml) was added to the extracted organic layer, to obtain triethyl ammonium (4-chloro phenylthio) difluoro methane sulfonate with a yield of 59%. The organic layer was concentrated using water and aqueous solution of sodium thiosulfate, filtered and dried, to obtain crude crystal which was recrystallized using ethyl-n-hexane acetate, to obtain triethyl ammonium (4-chlorophenyl thio) difluoro methane sulfonate in a yield of 32%.