The amorphous magnesium-bismuth alloys were prepared by the co-evaporation technique . The compositions of these alloys were determined from the rate of deposition of the two constituents using the quartz crystal oscillators. A new method to calculate the densities of the alloys from the rates of deposition is outlined. The electrical measurements of the d.c. resistivity shows that the Mg-Bi alloys close to the stoichiometric composition Mg3Bi2 has a very high resistivity. From the optical measurements, the stoichiometric Mg3Bi2 compound is found to have an optical band gap 0.264 + 0.006 ev. The dramatic changes in the optical band gap due to annealing make the proposed model densities of states of doubtful relevance.