The interaction of Al-1 wt% Si with a W-Ti barrier layer in the Al/Ti3W7/SiO2/Si system was studied over the temperature range of 400-500-degrees-C for reaction times up to 300 h. The interaction was found to be diffusion-controlled, and to occur in a layer-by-layer fashion. The first reaction product is always Al12W, which forms at the Al/Ti3W7 interface. With excess W in the system, Al will eventually be completely converted to Al12W, and further interactions result in the formation of an Al4W layer at the Al12W/Ti3W7 interface. The amount of Al4W increases at the expense of Al12W. Ti plays a minor role in the interaction and forms a small amount of Al3Ti precipitates in the Al12W matrix. Decomposition of the Ti3W7 pseudoalloy into W and Ti phases is not significant, and is not detected by X-ray diffraction even after annealing at 500-degrees-C for 300 h. The kinetics of the Al12W formation follows a parabolic reaction law with an activation energy of 2.53 eV. The sheet resistance of the film is insensitive to compound formation as long as a continuous Al film exists in the system. The sheet resistance increases dramatically when Al is consumed to the extent that it is no longer a continuous film. The sheet resistance of the Al12W layer is estimated to be 570 mOMEGA square-1.