We report the optical properties of a thin-film Mg2Si, which was successfully prepared as a continuous and homogeneous polycrystalline phase on a Si(100) face by an electrochemical process with a thickness greater than 10 km. The growth rate achieved in the process was the order of 10 mu m/h. Optical absorption coefficient spectra obtained at room temperature exhibited an absorption edge at around 0.8 eV Assignments of the peaks in the spectra to the optical transitions were based on using conventional reports. The absorption bands at 1.90 and 2.63 eV are consistent with the reported direct transitions in the reflectance measurements for single crystals and the results of band calculations. The absorption at the 14 band edge, which is claimed to be an indirect transition from the calculations, contains a substantial direct absorption. (C) 2007 Elsevier Ltd. All rights reserved.