The thermal behavior of diethyl biferrocenium tri-iodide has been reinvestigated with further detail in the range 250-300 K by Mössbauer spectroscopy. The data strongly suggests the onset of a localized-delocalized second-order transition, and are successfully reproduced by a cooperative adaptation of the vibronic Piepho-Krausz-Schatz model. The shape and amplitude of the expected contribution to the specific heat are discussed, as well as other spectroscopic data. © 1993 The American Physical Society.