The rates of the ultrafast intramolecular electron transfer (ET) for mixed-valence biferrocene monocation (Fe(II), Fe(III)) were determined in various solvents and at various temperatures by the H-1 NMR spin-lattice relaxation time measurements. The results of the ESR line broadening of biferrocene and ferricenium ions as well as the NMR relaxation measurements indicated that the correlation time responsible for the spin-lattice relaxation time of the cyclopentadienyl protons of biferrocene monocation was approximately equal to the lifetime of electron hopping between the two iron ions, i.e., the inverse of the rate constant of the intramolecular ET. The obtained ET rate constants exceeded 10(12) s(-1) at room temperature, e.g., 1.3 x 10(12) s(-1) in acetone at 298 K, and showed significant solvent dependence. The solvent and temperature dependences of the rate constants thus obtained were examined by theories considering the dynamic solvent effect. The obtained rate constants and the temperature dependences were respectively higher and smaller than those predicted by the theories.