Amorphous and crystalline Bi3.5Nd0.5Ti3O12 (BNT) thin films were deposited on Pt/Ti/SiO2/Si(100) substrates by a CSD processing. The optical parameters of BNT films were obtained by spectroscopic ellipsometry using the combination of a multilayer model and Bruggeman effective medium approximation. The dispersion of refractive index was analyzed by the Sellmeier oscillator model. The oscillator energy E-m and the dispersion energy E-d were calculated according to Wemple-Didomenico formula. Urbach absorption was found in both the amorphous and crystalline BNT films, which were different from the usual absorption in perovskite films. The mechanism of the Urbach absorption was ascribed to the substitution of neodymium for bismuth. The possible effect of Bi shortage in the crystalline film is also discussed. (C) 2004 Elsevier B.V. All rights reserved.