We synthesized a single phase GdSi2 film on a Si(111) substrate with an interfacial SiO2 layer. In order to take account of the role of the interfacial SiO2 layer, systematic investigations on clean and oxidized Si substrates were done by using in situ reflection of high energy electron diffraction, x-ray diffraction, and atomic force microscopy of the silicides formed with post annealing. Our result showed that the interfacial SiO2 layer enhanced the structural transformation of the initial GdSi1.7 hexagonal phase into the GdSi2 orthorhombic phase above the decomposition temperature of SiO2(∼800 °C). We proposed a reaction mechanism for the GdSi2 film formation with the help of the interfacial SiO2 layer. The measured electrical resistivity of the Gd-silicide film strongly depends on the silicide phase. © 2003 American Institute of Physics.