Bi2Te3 films were prepared by thermal evaporation technique. X-ray diffraction analysis for as-deposited and annealed films in vacuum at 150 T were polycrystalline with rhombohedral structure. The crystallite size is found to increase as the film thickness increases and has values in the range 67-162 nm. The optical constants (the refractive index, n, and absorption index, k) were determined using transmittance and reflectance data in the spectral range 2.5-10 mu m for Bi2Te3 films with different thicknesses (25-99.5 nm). Both n and k are independent on the film thickness in the investigated range. It was also found that Bi2Te3 is a high refractive index material (n has values of 4.7-8.8 in the wavelength range 2.5-10 mu m). The allowed optical transitions were found to be direct optical transitions with energy gap E-g(d) - 0.21 eV. The optical conductivities sigma(1) = f(hv) and sigma(2) =f(hv) show distinct peaks at about 0.13 and 0.3 eV, respectively. These two peaks can be attributed to optical interband transitions. (c) 2004 Elsevier B.V., All rights reserved.