The present invention provides novel fluorosulfonic acids and salts thereof having greater stability and higher acidity than conventional fluorosulfonic acids; and synthetic intermediates thereof. The invention also provides processes for producing novel fluorosulfonic acids from which various derivatives can be easily synthesized; and uses of these fluorosulfonic acids. Specifically, the invention provides a compound represented by formula (1): R--S(.dbd.O).sub.n--CF.sub.2--SO.sub.3H (1) wherein R is a group bound to an adjacent sulfur atom through a carbon atom, examples of R being an optionally substituted alkyl group, an optionally substituted haloalkyl group, an optionally substituted alkenyl group, an optionally substituted haloalkenyl group, an optionally substituted alkynyl group, an optionally substituted haloalkynyl group, an optionally substituted cycloalkyl group, etc.; and n is 1 or 2. The invention also provides salts of such a compound, etc.