The cyclopolymerization of isoprene was readily effected with the stable carbonium ion salts C7H7+SbCl6−and (C6H5)3C+SbCl6−in nitrobenzene. Little or no polymerization occurred when n-heptane, benzene, toluene, acetonitrile, methylene chloride, or nitromethane were used as reaction media. The molecular weights of the polymers obtained by light-scattering were in the range of 6,000–70,000 although the intrinsic viscosities in benzene were extremely low, i.e., 0.03–0.06. Triphenylmethyl groups were detected in cyclopolyisoprene prepared with (C6H5)3C+SbCl6−as catalyst. No tropylium residues were detected in polymers prepared with C7H7+SbCl6−. All polymers contained 10–45% unsaturation (trans-1,4 and 3,4). The unsaturation decreased with increasing trityl ion concentration but was independent of tropylium ton concentration. At constant isoprene concentration, a limiting conversion was reached; polymerization resumed on the addition of monomer. SbCl5in nitrobenzene was an effective catalyst for the cyclopolymerization of isoprene in the absence of cocatalyst. The proposed initiation mechanism involves the formation of a catalyst-nitrobenzene charge transfer complex which undergoes a one-electron transfer with isoprene monomer to yield a monomer cation-radical. The latter generates a fused ring structure by success ive cation and radical additions to monomer, as proposed for cyclopolymerizations initiated by C2H5AICl2and C2H5AlCl2-TiC14.