Monoclinic bismuth oxide (Bi2O3) films have been prepared by thermal oxidation of vacuum evaporated bismuth thin films onto the glass substrates. In order to obtain the single phase Bi2O3, the oxidation temperature was varied in the range of 423-573 K by an interval of 50 K. The as-deposited bismuth and oxidized Bi2O3 films were characterized for their structural, surface morphological, optical and electrical properties by means of X-ray diffraction, scanning electron microscopy (SEM), optical absorption and electrical resistivity measurements, respectively. The X-ray analyses revealed the formation of polycrystalline mixed phases of Bi2O3 (monoclinic, alpha-Bi2O3 and tetragonal, beta-Bi2O3) at oxidation temperatures up to 523 K, while at an oxidation temperature of 573 K, a single-phase monoclinic alpha-Bi2O3 was formed. From SEM images, it was observed that of as-deposited Bi films consisted of the well-defined isolated crystals of different shapes while after thermal oxidation the smaller dispersed grains were found to be merged to form bigger grains. The changes in the optical properties of Bi2O3 films obtained by thermal oxidation at various temperatures were studied from optical absorption spectra. The electrical resistivity measurement depicted semiconducting nature of Bi2O3 with high electrical resistivity at room temperature. (C) 2008 Elsevier B.V. All rights reserved.