The understanding and control of the transition from columnar to equiaxed growth (CET) is an essential issue according to the desired usage properties. In Al-based alloys, refiners are routinely added to favor equiaxed growth. In that case, equiaxed grains nucleate preferentially on the refiners and can compete with columnar grains. To predict CET, two main parameters have been studied in models: the particle density,N0 and the nucleation undercooling of the refining particles (TN).We have used DSC successfully to measure the critical value of the nucleation undercooling. Thanks to these results, the efficiency of the specific refining particles (Al–5.0 wt% Ti–1.0 wt% B) in refined Al–3.5 wt% Ni alloy has been proved. Additionally, these measurements clearly point out the lowest efficiency of the same particles in refined Al–7.0 wt% Si which is generally admitted to be due to a Si-poisoning effect. More generally, the nucleation undercooling measured can be used in models dedicated to the quantitative prediction of CET.