The resonance Raman spectra of the trans- and cis-1,3-pentadiene and the 2-methyl-1,3-butadiene (isoprene) radical cations generated by gamma-irradiation in a Freon glass are reported. The spectra are excited in resonance with the first absorption band. The spectra, which are interpreted with the help of density functional theory calculations, indicate the presence of both the s-trans and the s-cis rotamers of all three radical cations.