High purity (HP) Al-5 wt% Si with additions of Ta, TiB ₂, Eu and P were produced by arc melting and melt-spinning. Nucleation kinetics of entrained eutectic Si droplets in HP melt spun Al-5Si based alloys was investigated using differential scanning calorimetry (DSC), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). No significant effect of Ta and/or TiB ₂ on nucleation kinetics and microstructure of entrained eutectic Si droplets was observed, although Al ₃Ta was observed at hyperperitectic concentrations. In contrast, a significant effect of Eu and/or P on nucleation kinetics and microstructure of entrained eutectic Si droplets was observed. Increasing P concentration increases the onset temperature and exhibits a lower undercooling required for the nucleation of entrained eutectic Si droplets. Large Al ₂Si ₂Eu phase was observed both within the Al matrix and along the boundaries of entrained eutectic Si droplets, while small Al ₂Si ₂Eu phase was also observed within entrained eutectic Si droplets. With increasing P, EBSD reveals the transition from fine randomly multiply-oriented fibrous eutectic Si to coarse single-oriented plate eutectic Si within the entrained eutectic Si droplets. With further increasing P, fewer coarse Si crystals were observed within the entrained eutectic Si droplets. EBSD indicates that the entrained eutectic Si droplet can be nucleated from AlP nucleation site.