Two mixed-valence iron dimers, [(LFe2)-Fe-1(mu-OAc)(2)](ClO4) and [(LFe2)-Fe-1(mu-OBz)(OBz)(H2O)](ClO4), were investigated. The temperature dependence of the effective magnetic moments and the position and shape of the intervalence absorption bands were calculated using a Hamiltonian taking into account isotropic exchange interaction, double exchange, Zeeman interaction, zero-field splitting for the ground state and vibronic coupling with the PKS out-of-phase mode. The similarities and differences of the investigated compounds were explained on the basis of analysis of the adiabatic potentials. The behaviour of both systems is determined by a strong competition between two main processes: double exchange interaction and vibronic coupling with PKS out-of-phase mode. Degree of delocalization of the itinerant extra electron was calculated at different values of temperature.