In this study, trifluoromethanesulfonic acid was employed as catalyst for the cyclization of polyisoprene. From H-1 nuclear magnetic resonance spectroscopy and gel permeation chromatography investigations, cyclization was confirmed, and the molecular structure variation of polyisoprene during cyclization reaction was determined. With the comparison to that catalyzed by other strong acids (methanesulfonic acid), polyisoprene was cyclized with much higher efficiency, using trifluoromethanesulfonic acid as catalyst. (c) 2006 Wiley Periodicals, Inc.