The optical constant dispersions of ion-beam-synthesized Mg2Si phase in Si matrix are obtained from the transmittance and reflectance spectra. Two types of samples are studied - one of them with Mg2Si phase embedded in n-type (100)Si and the other with Mg2Si phase embedded in p-type (100)Si. The formation of the phase is proved by Raman scattering and infrared transmittance measurements. From the interpretation of the optical constant dispersions, the energies of the transitions nearby the material band edge are determined. As a result the band diagram of the heterojunction Mg2Si/Si is obtained. The results about the Mg2Si band gap value are compared with the theoretically predicted and experimentally determined ones. The value of the conduction band offsets Of Mg2Si and Si is not reported by now. (c) 2006 Elsevier B.V. All rights reserved.