4-Vinyl-1-cyclohexene was polymerized by radiation at room temperature under vacuum and in atmospheric air. The change of percent conversion with irradiation time is almost linear up to limiting conversion, but it is S-type in atmospheric air. It was shown that the mechanism of polymerization is most probably ionic rather than free-radical. The cationic polymerization was carried out in dichloromethane solution at -8-degrees-C using BF3(C2H5)2O as catalyst. The nature of the polymer obtained was investigated by UV, IR, and NMR spectroscopic methods, and the molecular weights by viscosity. Polymerization takes place by both linear addition of carbon-carbon double bonds and cyclopolymerization. The rate ratio of cyclization and linear propagation was calculated.