The interaction between an aluminium film and a W-Ti film during annealing was monitored by sheet resistance measurements. The sheet resistance of this thin-film stack is mainly determined by the thickness and resistivity of the aluminium film. During annealing, interaction takes place between the two films, resulting in an increase in sheet resistance of the thin-film stack. This increase in sheet resistance can be caused by an increase in aluminium resistivity, due to the dissolution of tungsten and titanium in the aluminium film, and by a decrease in aluminium thickness, owing to the formation of aluminium-tungsten and aluminium-titanium compounds. By measuring not only the sheet resistance but also the temperature derivative of the sheet resistance, the resistivity and the thickness of the aluminium film can be obtained separately. It is shown that the large increase in sheet resistance during the first stage of the thin-film interaction is mainly caused by the large resistivity increase of the aluminium.