Systematic ellipsometric studies of MoSi 2 (tetragonal) thin films formed by RTA processing of cosputtered MO 2 5 Si 7 5 , Mo 3 0 Si 7 0 , and Mo 3 6 Si 6 4 thin films are discussed. The optical properties of these films in the measured spectral range 1.3-5.3 eV were observed to be dominated by the microstructural variations such as due to the changes in density, oxide overlayer thickness and composition, surface roughness, and redistribution of available excess silicon (after formation of MoSi 2 (tetragonal) phase). These microstructural variations indicated modification of interfaces and significant change in conductivity, which were corroborated in the AES depth profiles and the electrical resistivity measurements.