Low-temperature-stable sulfonium ion adducts were generated by addition of mono- and disulfides to TiCl4-catalyzed quasiliving polyisobutylene (PIB). The adducts were studied in situ via low temperature NMR in 50/50 (v/v) CS2/CD2Cl2 using the initiator 2-chloro-2,4,4-trimethylpentane (TMPCI) and C-16 and C-20 tertchloride oligo-isobutylenes as models for the PIB chain end. At temperatures less than or equal to -60 degrees C, quantitative 1:1 adducts were formed between the (di)sulfides and TMPCI or the oligo-isobutylenes. Adduct formation prevented further homopolymerization of isobutylene, but when a more reactive nucleophile such as an alcohol or amine was added to the reaction, the adducts were destroyed. Both elimination and substitution products were obtained at the PIB chain end. With PIB-monosulfide adducts, elimination was the principle decomposition pathway, and near-quantitative formation of exo-olefin PIB was achieved upon termination by a hindered tertiary amine, such as proton trap, 2,6-di-tert-butylpyridine. For PIB-disulfide adducts, decomposition was observed to occur principally through sulfur-sulfur cleavage, yielding useful thioether end groups, e.g., when terminated with triethylamine, the PIB-bis(2-bromoethyl) disulfide adduct yielded near-quantitative primary bromide-terminated PIB.