[en] An e.s.r. study of the radicals produced by low temperature γ-irradiation of dicyclopentadiene and 1-isopropylidenetetrahydroindene in an adamantane-thiourea matrix leads to the identification of the species dicyclopentadien-9-yl and 1-(tetrahydroinden-1-ylidene)ethyl respectively. The origin, structure, and e.s.r. properties of these radicals are discussed on the basis of the results of progressive thermal annealing experiments and of SCF MO calculations performed to the INDO and HMO (McLachlan) levels of approximation. (author)