Diffusing species in the reaction of thin tungsten films with aluminum overlayers in vacuum have been investigated by marker experiments using Rutherford backscattering spectrometry and scanning electron microscopy techniques. It has been found that Al was the dominant diffusing species in the reaction. Ni has been used as the marker material. Samples with both pure and Al‐diluted and both unburied and buried Ni markers at the interface of thin W films and Al overlayers have been studied to compare barrier effect of the markers. Our results showed that the Ni‐based markers do not impede the reaction of thin W films with Al overlayers.