The effect of the addition of Si to an al alloy on the reaction morphology of al12W in Al-Cu-Si/Ti-W bilayers was studied with plan-view and cross-section transmission electron microscopy (TEM). The addition of 0.5 wt.% Si to an Al-0.5Cu alloy film increases multiple spiked growths of the al12W compound by the reaction of the Al-0.5Cu-0.5Si film with the Ti-W sublayer after heat treatment at 450°C for 30 min. Increasing the Cu and Si content from 0.5 to 1.5% significantly reduced the spiked growth of al12 into the al-1.5Cu-1.5Si film. However the spike number density remained high compared to the reaction of a binary Al-1.5Cu film with the Ti-W layers. The results indicates that the addition of Si to form a ternary Al-Cu-Si film results in a more irregular, less planar Al12W morphology. This morphology can have a detrimental effect on thermal stability and electromigration resistance.