Chlorination of a thermoplastic styrene-butadiene-styrene rubber (SO) with different amounts of trichloroisocyanuric acid (TCI) solutions in ethyl acetate improved its adhesion to polyurethane adhesives. A strong interaction of the PU (polyurethane) adhesive and the chlorinated SO rubber chains is produced at the interface. The increase in the concentration of TCI from 0.5wt% up to 2-7wt% resulted in the deposition of crystallites of unreacted TCI on the rubber surface. The remaining TCI on the rubber surface migrates through the PU adhesive producing some chlorination of the PU chains. The failure of the joint is located in this interface composed of both chlorinated SO rubber and partially-chlorinated PU adhesive.