To model the thermodynamic properties of mixed-valence molecular solids, we have considered a set of coupled bistable oscillators: the oscillators are described by a vibronic Piepho-Krausz-Schatz Hamiltonian, and are coupled to each other by asymmetric potential terms. The mean-field treatment of the problem leads to a continuous transiton, from a localized to a delocalized state as the temperature increases, with an associated discontinuity of the specific heat; the thermal evolution of the latter significantly depends on the value of the intramolecular transfer integral. Numerical values have been considered in relation to the situation of biferrocenium salts. © 1993.