Poly(spiro[2,4]hepta-4,6-diene) (PSHD) was exposed to oxidants such as bromine, iodine and antimony pentachloride either in solution or as a film. Bromine was rapidly added to the C=C bonds of the polymer. Heating the brominated polymer to elevated temperatures resulted in dehydrobromination and the formation of a conjugated, unsaturated polymer backbone, recognizable by the formation of a black, insoluble product. Exposure of PSHD to iodine or antimony pentachloride led to the formation of a black product with a metallic appearance. The conjugation length of the resulting oxidized polymer was rather short. From optical absorption spectra a length of six to eight C=C bonds could be deduced. Infra-red spectra indicated that oxidation was accompanied by a cleavage of the cyclopropyl groups. The oxidized polymer was electrically conducting. Maximum conductivities of 10(-5) S cm-1 were obtained when PSHD was exposed to iodine vapour for several hours. From electron spin resonance studies it could be derived that the conductivity is due to an electron-hopping process.