We analyzed successfully the refractive index, n, and extinction coefficient, k, of three samples such as as-deposited single-phase anatase-TiO2 polycrystalline thin films on slide glass substrates (#1), the sample surface-treated by N-2-H-2 mixed-gases plasma (#2), and the sample being additionally anneal-treated in N-2 gases (#3), by spectroscopic ellipsometry (SE). The double-layered film configuration named as the surface and the bulk layers is employed in SE analysis. The optical properties of the surface layer are confirmed to be important to explain the significant red shift of the absorption edge of both surface-treated samples observed in the preceding report. Particularly, the obtained imaginary part of complex dielectric constant of the surface layer of the samples #2 and #3 shows significant difference from that of the sample #1. This is due to the absorptive transitions not by solely anatase-TiO2 but by mixed formation of TiO2-xNx and TiN with metallic characteristics. (c) 2004 Elsevier B.V. All rights reserved.