Abstract The ESR spectra of the 9,10-dihydrophenanthrene radical anion change drastically with the temperature, and at temperatures around −30°C the spectra show a marked line-width alternation effect. These temperature-dependent ESR spectra and the line-width alternation effect have been interpreted in terms of the conformational interconversion of the 9,10-dihydrophenanthrene radical anion molecule. From an analysis of the spectra, the activation energy of the inversion has been estimated to be 6.3±0.6 kcal/mol. It has also been estimated, from the ratio of the axial and equatorial proton splittings, that the dihedral angle of the axial protons is about 11°. These data are compared with those of the 4,5,9,10-tetrahydropyrene radical anion. The ESR of the 4,5-dihydropyrene radical anion is also presented.