The polycrystalline MgB2 thin films were prepared on Al2O3 and Si substrates. Their optical properties in the broad spectral range (30-110 000 cm (-1)) have been studied by the normal incidence reflectance and ellipsometry. The model consisting of a surface oxide layer and MgB2 bulk provides consistent results. The optical constants were determined by fitting the ellipsometry data and the Kramers-Kronig analysis of the reflectance data. The plasma frequency of MgB2 has been determined from the optical constants. (C) 2004 Elsevier B.V. All rights reserved.