Abstract The ENDOR spectra of several alkylated biphenyl anions With para- and meta-substitutions were observed, and the relation between the molecular size and the optimum temperature of ortho-proton ENDOR were investigated in terms of the electron-nuclear dipole-dipole interaction. On the basis of the Stokes-Einstein model, a linear relation was found between the inverse of the effective radius and the cubic root of the η/T value at the optimum temperature of the ortho-proton ENDOR, under the assumption that the radical anion undergoes rotation accompanied by the solvent shell. In this situation, the separation between the radical anion and the counter cation thus estimated is more than 8.3 Å and the ion pair exists as a solvent-separated ion pair.