Radical anions of three perfluorocycloalkanes and several halogenofluoromethanes have been detected and identified by e.s.r. studies following γ irradiation at 77 K of solid solutions containing up to 5 mol % of the parent compound in neopentane or tetramethylsilane. The isotropic e.s.r. spectra of c-C3F–6, c-C4F–8 and c-C5F–10 are photobleached by visible light and show the second-order structure characteristic of 6, 8 and 10 equivalent fluorines, respectively, the total 19F coupling being approximately the same value (1170 ± 20 G) in each case. Identical e.s.r. spectra were generated in photoionization experiments using tetramethyl-p-phenylenediamine, confirming the radical anion identifications. The equivalence of the fluorines indicates that the unpaired electron is delocalized over the entire molecular framework in an orbital of high symmetry. The e.s.r. spectra of the CF3X– radical anions (X = Cl, Br, I) were anisotropic and showed clear evidence for axially symmetric hyperfine interactions with three equivalent fluorines and the unique halogen. On this basis, a matrix diagonalization program was used to calculate the line positions and the best-fit e.s.r. parameters obtained. Confirmation of the CF3X– identifications was achieved by parallel photoionization experiments and by studies showing that the decay of the CF3X– spectrum in neopentane above 100 K was accompanied by a growth in the spectrum of the CF3 radical. The spin density distributions calculated from the e.s.r. parameters of these congeneric radical anions suggest that the unpaired electron resides in an a1(σ*) antibonding orbital which is composed largely of the p orbitals from carbon and the unique halogen which lie along the C3v symmetry axis of the radical anion. Consistent with this proposal, the spin densities in the s and p oribtials of the unique halogen increase along the series Cl, Br, I, which is the order expected for the effect of decreasing halogen electronegativity.