Optically active strontium substituted barium titanate (BaxSr1-xTiO3) thin films have been prepared on p-silicon and glass (Corning 7059) substrates by RF magnetron sputtering in a flowing Ar/O-2 atmosphere. Transmittance and ellipsometric study of the films show that the refractive index is a function of the film composition. In the W-visible transmittance spectra, the band edge absorption of BST films shifts to lower energies when the material is in the crystalline state rather than the amorphous state. Optical band gap calculated from UV-visible transmittance spectra increases with the increasing strontium concentration. Extinction coefficient of the films in visible region is in the order of 10(-3). The infrared (IR) spectra with dominant absorption bands due to Ti-O stretching, Ti-O bending and (cation)-TiO3 vibrations suggest the formation of a single-phasic perovskite BST film in magnetron sputtering.