The presence of polyene sequences, introduced into polyisoprene by interaction with molecular iodine in the solid phase and in chloroform solution has been established using fluorescence emission and excitation spectroscopy. Changes in the electrical conductivity of polyene films following the interaction have been measured and the mechanism of polyene formation is discussed. The results support the conclusions of other workers that the presence of conjugated sequences are a pre-requisite for conducting materials rather than the intersite hopping between carbocation centres derived from oxidation of isolated double bonds which has been suggested.