The selective and quantitative sulfonation of polybutadiene (PB) and triblock copolymers polystyrene-b-polybutadiene-b-polystyrene (SBS) has been successfully obtained through a novel procedure consisting of two reaction steps carried out in a single batch: free radical addition of thiolacetic acid to olefinic double bonds followed by autocatalytic oxidation of the thioacetylated polymer with in situ generated organic peracids. The polymers resulting from the two reactions were fully characterized by NMR, FTIR, and elemental analysis; the ion exchange capacity (IEC) of the sulfonated polymers was assessed by elemental analysis and titration with NaOH. Critical desulfonation temperatures of the poly(alkylsulfonate)s were observed in the range 160-270 degrees C, whereas the thermal decomposition starts at about 380 degrees C. The TUNA-AFM analysis of a sample of SBS containing 79 mol % of alkylsulfonic groups showed a phase-separated morphology consisting of polystyrene domains of about 28 nm embedded in the sulfonated polymer matrix.