Photoinitiated substitution complexes of [M(CN)(8)](4-) (where M= Mo(IV) or W(IV)) and ethylenediamine with chromium(III) have been synthesized and characterized. On the basis of elemental analysis, the complexes have been assigned formulas as follows:Cr[Mo(CN)(C2H8N2)(OH)(6)]. 2H(2)O I for Mo(IV); andCr[W(C2H8N2)(O-2)(OH)(3)]. H2O II for W(IV)The characteristic IR absorption peaks for different entities present support the assigned formulas. Complex I shows the absorption peaks due to nu(C=N) stretching bond, NH asymmetric deformation, NH symmetric deformation and NH3 rocking mode showing the presence of cyanide and ethylenediamine. The loss of the absorption peak due to nu(C=N) stretching in the spectra of complex II confirms the substitution of all the cyanide ligands. The thermal degradation of the complexes has been studied by TG and DSC techniques. Both the complexes have a similar thermal decomposition behavior : involving expulsion of water molecules in the first step followed by the expulsion of other ligands. Kinetics and a thermal decomposition mechanism have been proposed for each complex. Thermodynamic parameters such as activation energy (E-a), pre-exponential factor (A) and entropy of activation (Delta S-#) have been calculated for each step, employing different integral methods of Doyle, Coats and Redfern, and Arrhenius. The reaction enthalpy is obtained from DSC data