The grain refining behaviour of a slightly hypereutectic binary Al-Sc alloy has been compared to a Al-Sc-Ti alloy, containing the same total concentration of solute. Grain refinement was generally more effective in the Ti containing alloy. In the binary Al-Sc alloy grain refinement occurs by small primary particles of the Ll(2) Al(3)Sc phase forming in the melt and acting as heterogeneous nucleants for alpha-Al. However, due to the low growth restriction factor of Sc, recalescence results in only 1 in 50 particles becoming viable nuclei for AI grains. In contrast, nearly 100% nucleation efficiency was observed in the ternary Al-Sc-Ti alloy. This large increase in nucleation efficiency is primarily caused by Ti greatly restricting the spherical growth rate and allowing more nucleants to become active before recalescence, although an important contributory factor is that it is likely that the Al-Sc-Ti alloy undergoes a peritectic reaction that sharpens the onset of nucleation.