Solid state thermal investigation and electron transfer reaction of a dinuclear complex [(phen)2MnII(CN)2FeIII(CN) 3(NH3)] · 3H2O (1) has been carried out. Compound 1 was synthesized and characterized by elemental analysis, IR spectra, electronic spectra, magnetic moment and thermal analysis. Heating in an inert atmosphere at 180°C compound (1) converted to [(phen)2MnIII(CN)2FeII(CN) 3(NH3)] (2) with a sharp colour change from light yellow to reddish brown. This reaction was also observed in by DTA an endotherm with no associated mass loss (by TGA) curve was observed. The rate constant of the transformation was evaluated from DTA results at different temperatures. Activation energy and enthalpy change were also evaluated. The reaction process is irreversible. X-Ray powder diffraction patterns were recorded for (1) and (2) which are almost identical. We were unable to synthesize a single crystal of either compound (1) or (2) for lack of solubility in most of the solvents.