Thermally induced electron-transfer reactions of dinuclear complexes of the composition [L2MnII(H2O)2][L2Mn II(CN)2FeIII(CN)4] 2·6H2O, where L = 1,10-phenanthroline 1 or 2,2′-bipyridyl 2, have been carried out. Both the complexes have been synthesized and characterized by elemental analysis, IR, electronic, EPR, Mössbauer spectra and thermal analysis. Heating in an inert atmosphere at 210 C for complex 1 and at 190°C for complex 2 caused conversion to [MnII(Phen)2(H2O)2][(Phen) 2MnIII(CN)2 FeII(CN)4]2 1a and [MnII(bipy)2(H2O)2][(bipy) 2MnIII(CN)2FeII(CN) 4]2 2a with a sharp colour change from yellow to reddish brown in each case. Thermal investigation of complexes 1 and 2 under non-isothermal conditions in a dynamic inert atmosphere showed endothermic peaks in DTA curves at 210 and 190 C, respectively, whereas no mass loss in the corresponding TGA curves was observed. The rate constant of the electron transfer has been evaluated from DTA curves at different temperatures. Activation energy and enthalpy change were also evaluated. This process is irreversible as no peak appeared in the DTA curve on cooling. X-ray powder diffraction patterns were recorded and showed that 1, la and 2, 2a are almost identical. We were not able to synthesize single crystals of the complexes 1 and 1a, 2 and 2a due to the lack of solubility in most of the solvents. Copyright © 1996 Elsevier Science Ltd.