The grain refining effect of four peritectic-forming solutes (Ti, V, Zr and Nb) as well as three eutectic-forming solutes (Cu, Mg and Si) on pure Al was investigated. Significant grain refinement is observed by the addition of peritectic-forming solutes, whereas the addition of eutectic-forming solutes only slightly decreases the grain size. The mechanisms underlying the grain refinement of these alloys were then studied by a new analytical methodology for assessing grain refinement that incorporates the effects of both alloy chemistry and nucleant potency. It is found that the low degree of grain refinement by the addition of eutectic-forming solutes is mainly attributed to the segregating power of solutes, i.e. the constitutional undercooling contribution. However, peritectic-forming solutes do not only cause grain refinement by their segregation power but, more importantly, they introduce copious potent nuclei into the melt and promote significant grain refinement via heterogeneous nucleation.