Methyl phenyl sulfoxide was found to undergo self-polycondensation in triflic acid in the presence of diphenylamine to produce poly(methylsulfonio-1,4-phenylene triflate). Diphenylamine formed a charge-transfer (CT) complex with the hydroxy methyl phenyl sulfonium cation produced by the protonation of the sulfoxide, facilitating the electrophilic substitution reaction of the sulfonium cation onto the carbon atom of the terminal phenyl ring. The obtained polymer is a highly crystalline material whose structure was determined by X-ray analysis using the typical geometric parameters of the sulfoniophenylene dimer and trimer model compounds. The orthorhombic unit cell of the polymer (a = 10.875 Angstrom, b = 10.449 Angstrom, and c = 18.629 Angstrom) contains eight monomeric units. The space group is Pbcn (#60). Two molecular chains along the c axis pass through the unit cell, one through the corner and the other through the center. The crystal structure of the polymer not only revealed a structural relevance to those of oxy- and thiophenylene polymers but also provided support for CT complex formation between the polymer and diphenylamine.