in order to explore the alleged negative effects of Si on precipitation of Al3Sc a series of 5 Al-Sc-Si alloys were made: Al 0.4 wt.%Sc x wt.%Si where x is either 0, 0.2, 0.4, 0.6 or 0.8. The alloys were subjected to heat treatments corresponding to either dispersoid formation (500degreesC) or to precipitation hardening (250degreesC, 300degreesC). The response to the heat treatments was characterised by hardness measurements, scanning electron microscope and transmission electron microscope. It was found that a portion of the Sc is readily precipitated in the as-cast material, both by continuous precipitation and by discontinuous precipitation. Coherent Al3Sc precipitates were found in all of the alloys. The hardness of the as-cast material increased with increasing Si content. Heat treatment at 500degreesC led to a dense Al3Sc distribution in the binary Al-Sc alloy. For the Si-containing alloys an Al3Sc dispersoid-free zone near the grain boundaries was observed. The width of the zones increased with increasing Si content and in the alloy with 0.8 wt.% Si no Al3Sc dispersoids were observed at all. Other phases were observed primarily at the grain boundaries. Heat treatments at 250degreesC and 300degreesC led to precipitation hardening in all of the alloys. The hardness increase from precipitation decreased with increasing Si content. However, the alloys with high Si content had higher as-cast hardness. Thus, all alloys achieved approximately the same hardness after precipitation hardening. For overaged specimens there was a slight tendency for decreasing hardness with increasing Si content in the alloys.