(Pb1−xLax)Ti1−x/4O3 (PLT) ferroelectric thin films with various La concentrations have been grown on LaNiO3/Si(100) substrates by a modified sol-gel technique. X-ray-diffraction analyses show that the PLT and LaNiO3 thin films are polycrystalline and entirely perovskite phase. The infrared optical properties of the PLT thin films have been investigated using infrared spectroscopic ellipsometry in the wavelength range of 2.5-12.5 μm. By fitting the measured ellipsometric parameter (tanΨ and cos Δ) data with a four-phase model (air/PLT/LaNiO3/Si), and a derived dispersion relation for the PLT thin films, the optical constants and thicknesses of the thin films have been determined. The refractive index of the PLT thin films decreases with increasing wavelength; however, by a Kramers-Kronig analysis the extinction coefficient increases with increasing wavelength. Moreover, the refractive index and extinction coefficient of the PLT thin films increase with increasing La concentration. This indicates that the infrared optical constants of the PLT thin films are a function of the La concentration. It is believed that the increase in the infrared optical constants of the PLT thin films with increasing La concentration is mainly due to the crystallinity and the electronic band structure of the PLT thin films.