Lead lanthanum zirconate titanate (PLZT) thin films with different La concentrations (x), whose composition is x/40/60, have been grown directly on Pt/Ti/SiO2/Si substrates by a modified sol-gel method. x-ray diffraction analysis shows that the PLZT thin films are polycrystalline with a single perovskite p hase. The infrared optical properties of the PLZT thin films are investigated us ing the infrared spectroscopic ellipsometry in the spectral range of 2.5—12.6μ m. By fitting the measured ellipsometric parameters, and a classical dispers ion relation for the thin films, the optical constants and thickness of the thin films have been obtained. The refractive index of the PLZT thin films decreases with increasing La concentration; however, the extinction coefficient increases with increasing La concentration except for the PLZT (4%) thin films. It is bel ieved that the decrease in the refractive index for the PLZT thin films with inc reasing La concentration is mainly due to the crystallinity and the electronic b and structure of the PLZT thin films. The absorption coefficient of the PLZT thi n films is larger than that of the PZT thin films. The values of the effective s tatic charge in the PLZT thin films, which are smaller than those of the purely ionic materials for the PLZT thin films, decrease with increasing La concentrati on. This indicates that PLZT belongs to a mixed ionic-covalent compound.