Sensitized radiation-induced polymerization of indene monomer was achieved at a dose rate of 3 kGy/h. The sensitizer (1,1,2,2-tetrachloroethane or TCE) leads to higher polyindene yields and faster polymerization kinetics with respect to bulk radiation-induced polymerization of indene. The radiation chemical yield Gp was found to increase with the dose in sensitized polymerization of indene following a power law, while an opposite trend was detected in the absence of the sensitizer. The sensitizer enhances the cationic polymerization mechanism in parallel to the free radical mechanism, as shown with both electronic absorption spectroscopy and FT–IR analysis of the polyindenes. Despite the enhancement of the polymer yield and the faster polymerization kinetics offered by the presence of TCE, the molecular weight of the resulting polyindene was found to be rather low. This was true whether the molecular weight was measured by end group analysis using X-ray fluorescence or the glass transition temperature determination with respect to the polyindenes produced with γ radiation without the sensitizer or with a pure cationic mechanism.