Abstract The ESR spectra for the cation radicals of CH3CH2F (FE) and its partially deuterated derivatives, formed by irradiation with γ-rays in SF6 matrices at 77 K, were observed at low temperatures. The spectra consist of two lines typical of an anisotropic hyperfine coupling to a 19F nucleus. The optimized geometry of FE•+ was obtained by the ab initio MO method, and hyperfine splittings were evaluated by density functional theory (DFT). Two structures, A and B, were obtained having 2A′ and 2A′′ electronic states, respectively. Structure A is in a lower energy state by 0.31 eV than structure B, and is assigned to the detected cation radical. This structure has a SOMO composed of the 2p orbitals of two C and an F atoms in the molecular plane, and has an elongated C–C bond of 0.1884 nm. This resembles the SOMO given for C2H6•+. Furthermore, the structural distortion of FE•+ in zeolite and the thermal dissociation of FE•+ in matrices are reported.