The electron transfer properties of the mixed-valence biferrocenium cation are investigated using the Piepho-Krausz-Schatz (PKS) vibronic theory. Parameters of the model are deduced from both vibrational spectroscopy (stiffness constant) and MO calculations (transfer integral and vibronic coupling parameter); the computed asymmetric distortion agrees with structural data; a specific heat Schottky anomaly, recently reported, is also explained in the frame of that model. The low-temperature transfer rate is also computed. it ranges from 1.5 × 1010 to 8.7 x 1012 s-1, according to the nature of the HOMO, and definitely lies higher than the Mössbauer timescale (≈ 108 s-1). © 1993.