Mg2Si is a semiconductor with a band gap previously reported to be in the range 0.6-0.8 eV. In spite of potential optoelectronic applications in an important infrared range, the growth of Mg2Si thin films on silicon substrates has received scant attention. This may be due to the difficulty of preparing Mg2Si in thin-film form. We find that intended reactive deposition of magnesium onto a silicon substrate, at temperatures from 200 to 500 degrees C, results in no accumulation of magnesium. However, codeposition of magnesium with silicon at 200 degrees C, using a magnesium-rich flux ratio, gives a stoichiometric Mg2Si film. The amount of magnesium which accumulates is determined by the total amount of silicon which was codeposited; the excess magnesium in the flux does not condense. Measurements of the optical transmittance of thin films thus obtained reveal an absorption edge. Extraction of the:absorption coefficient from the data,and analysis of its energy dependence, suggest an indirect band gap of similar to 0.74 eV, plus direct transitions at similar to 0.83 and similar to 0.99 eV.