[en] The selectivity of formation of primary radicals in irradiated solid hydrocarbons and related polymers is analyzed on the basis of ESR studies at low and super-low temperatures. The first direct experimental evidence for selective localization of the primary alkyl radicals at conformational defects (gauche conformers) was obtained for high-density oriented polyethylene irradiated at 15K. This result implies highly selective primary C-H bond rupture, which correlates with the structure of the ionized molecules of linear alkanes. The correlation between the spin density distribution in the primary hydrocarbon radical cations and the initial mode of radical formation in the corresponding polymers is discussed. (author)