A novel surface reaction for the reduction of MgCl2 by AlEt3 in the presence of Au is reported. MgCl2 submonolayer and multilayer films are prepared by gas-phase deposition over polycrystalline gold in UHV. When submonolayer MgCl2 films are exposed to liquid AlEt3, the halide is reduced, with the formation of Mg clusters or islands, covered with hydrocarbon fragments. The reduction of the halide at the Au surface is faster, and more complete, when MgCl2 and AlEt3 molecular beams are simultaneously directed at the metal surface. The reduced Mg surface reacts promptly with gas-phase TiCl4, and a TiCl2/TiCl4 film is thus deposited. This experiment demonstrates the feasibility of producing a MgCl2 support with a large fraction of its surface capable of TiCl4 chemisorption. Thd possibility of synthesizing Ziegler-Natta catalysts with a large density of active sites is discussed.