A novel diffusion ’’marker’’ has been used in the backscattering study of the formation of Mo and W silicide films. Because of their closely similar crystallographic and chemical characteristics, Mo and W may be regarded as equivalent atoms in a diffusion process. Hence, in the formation of WSi2and MoSi2by interaction of a bilayer film of W+Mo with substrate Si, the interface between the W and Mo (observable by backscattering) becomes a ’’marker’’ to permit identification of the moving species (Si atT<1000 °C).