Photosubstituted mixed ligand complexes of Co(II) with irradiated solutions of Mo(CN)(8)(4-) and W(CN)(8)(4-) with ethylenediamine have been synthesized. The complexes have been isolated and characterized by their elemental analysis and IR spectroscopy. The assigned formulae are Co-2[Mo(CN)(2)(C2H8N2)(2)(OH)(2)]Cl-4.2H(2)O I and Co[W(CN)(2)(C2H8N2)(2)(OH)(2)]- Cl-2.2H(2)O II for Mo(IV) and W(IV) complexes, respectively. The IR peaks observed for both the complexes show N-H stretching, H-N-H bending and stretching vibrations of the C-N and C-C bonds. Depending on the nature of the absorption bands, complex I is assigned as -trans and complex II as -cis configuration. The thermal decomposition of these complexes has been studied by TG and DSC techniques. The reaction scheme for decomposition of each complex has been proposed. The activation energy (E-a), pre-exponential factor (A) and entropy of activation (Delta S-#) have been calculated by employing the integral methods Like Arrhenius, Coats-Redfern and Doyle. Enthalpy (Delta H) values for each transition are obtained from DSC data.