Poly(styrene-b-isobutylene-b-styrene) block copolymers made through controlled-living cationic polymerization using the Bu-t-m-DCC-TiCl4 initiating system in 60/40 (v/v) hexane-methyl chloride cosolvents were analyzed using curve-resolution software in conjunction with high-resolution g.p.c. Fractional precipitation and selective solvent extraction were applied to a representative sample in order to confirm the identity of contaminating species. The block copolymers were found to consist of 70-75 wt% of the target molecule, and about 20 wt% of coupled (and higher Linked) block copolymers formed by intermolecular electrophilic aromatic substitution linking reactions occurring during the styrene polymerization. Minor contaminants were identified as diblock copolymer and low molecular weight polystyrene homopolymer.