Polycrystalline Bi4-xLaxTi3O12 (BIT, x=0, 0.5, 0.75, and 1.0) thin films have been grown on Pt/Ti/SiO2/Si substrate by metalorganic decomposition method at 650 degrees C. The studies of X-ray diffraction patterns and atomic force microscopy images indicate that the crystallization of BLT films was affected by the La substituting concentration. The refractive index and extinction coefficient of BIT thin films were determined by fitting the infrared spectroscopic ellipsometric data using a classical dielectric function formula. In the wavelength range of 2.5-8.0 mu m, as the La concentration increases, the refractive index decreases. The refractive index of BIT thin films in the wavelength range of 400-1100 nm, derived from the reflectance spectra, decreases with increasing La concentration. The La concentration dependence of optical constants for BIT films was investigated. (C) 2008 Elsevier B.V. All rights reserved.