formula i the sulfone compound represented by the general formula ii can be obtained by reacting for example bisphenol s with benzenedisulfonyl halide in this case too the reaction is carried out at an elevated temperature in an organic solvent as is the same with the aforesaid reaction between bisphenol s and benzenesulfonyl halide and extraction of the reaction mixture with an organic solvent yields a reaction product containing as a major component the sulfone compound of the formula ii the molar ratio of bisphenol s to benzenedisulfonyl halide is in the range of from 5 1 to 1 5 which is properly selected depending upon the kind of the end product the sulfone compound according to the present invention may be suitably used as a sizing agent for reinforcing fibers in the form of a solution in an organic solvent such as acetone methyl ethyl ketone or a halogenated hydrocarbon or in the form of an aqueous emulsion using a surfactant one or more aids commonly added to known sizing agents may be used if desired the sizing treatment for the deposition of the sizing agent onto reinforcing fibers may be effected by a dipping process a spraying process a process using a roller or any other suitable process reinforcing fibers to be treated according to the present invention include for example carbon fibers glass fibers boron fibers ceramic fibers and metal fibers both continuous and chopped fibers may be.