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Silver (Ag) inoculation was found to significantly reduce the average grain size of cast zinc (Zn) by up to 90%. The mechanism of such grain refinement in cast Zn was investigated through varying the addition level of this peritectic-forming solute. The reduction in grain size was sensitive to Ag content due to its large growth restriction factor. When the Ag content was over its maximum solid solubility in Zn, the in situ formed nucleation particles, with dendritic morphology (different from the previously reported faceted/spherical shape), were reproducibly observed near the grain centres in the refined alloys. These particles were determined to be pro-peritectic AgZn₃ phase based on the information from the Zn–Ag phase diagram, results from thermal analysis, phase identification and chemical composition. The high potency of the in situ formed AgZn₃ particles, as nucleation sites for Zn grains, was further verified by …