The Pb1-xLaxTiO3 (PLT) ferroelectric thin films on Pt/Ti/SiO2/Si(1 0 0) substrates were fabricated by a sol-gel process. The infrared optical properties of PLT thin films with x = 0.1, 0.2, 0.25 on platinized silicon Pt(1 1 1)/Ti/SiO2/Si(1 0 0) substrates were studied by infrared spectroscopic ellipsometry (IRSE) technique in the wavelength region from 2.5 to 12.6 mum. The infrared optical constants were obtained by fitting the IRSE data using a classical dielectric function model. The refractive index increases, and the extinction coefficient decreases as wavenumber increases in the measured infrared region. The refractive index of PLT thin films decreases as the La contents increase in the films. (C) 2003 Elsevier B.V. All rights reserved.