The dealkylation of sulfonium salts with nucleophiles such as halide ions takes place effectively to yield poly(phenylene sulfide). The kinetic analysis by means of UV-vis spectroscopy reveals that the dealkylation proceeds quantitatively to convert the sulfonium bond to a thioether bond. The reaction rate constants of the dealkylation by the halide ions are in the order I- > Br- > Cl-. A propyl substituent on the sulfonium salt results in a lower dealkylation rate than that with methyl substituents, because of the steric hindrance of the propyl group. The demethylation of poly [methyl-(4-(phenylthio)phenyl)sulfonium trifluoromethanesulfonate] also proceeds efficiently on soaking thin films in dichloromethane solution containing tetraethylammonium halide for 20 h and results in the formation of a transparent and amorphous poly(p-phenylene sulfide) (PPS) film, because the synthetic procedure occurs under the glass transition temperature of PPS (90 degrees C), although PPS is well-known to be a highly crystalline polymer.