For Bi2O3 thin films, the influence of preparation conditions on the optical absorption, in the spectral range of 280-1260 nm, was investigated. Bi2O3 films (d = 0.07-1.50 mum) were prepared by thermal oxidation, in air, of vacuum evaporated Bi films. Glass and mica substrates were used. Structural investigations (performed by X-ray diffraction and optical polarizing microscopy) showed that films have a polycrystalline and multiphasic structure. The values of energy gap, E-g (direct transitions), calculated from the absorption spectra, ranged between 2.40 and 3.96 eV. The values of E-g for Bi2O3 films are strongly influenced by the heat treatment characteristics, which determine the pi;ase composition of samples.