The electrical and optical properties of 1,4-cis-polyisoprene before and after doping with SnCl4 and SbCl5 were investigated. The material becomes reddish brown and black upon doping with SnCl4 and SbCl5, respectively, and the conductivity increases by about 11 orders of magnitude. The optical absorption, infrared spectra, elemental analyses, and the studies of reversibility upon compensation indicate that radical cation formation is the most likely mechanism for electrical conduction in these complexes. The results are consistent with previously published articles involving iodine doping. (c) 1994 (c) John Wiley & Sons, Inc.