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The poisoning effect of excess Si solute on the grain-refining potency of Al–Ti–B grain refiners in Al–Si casting alloys has been studied in a crystallographic investigation. The edge-to-edge matching model was used for investigating and comparing the possible poisoning effects of several binary and ternary intermetallic compounds containing Si and Ti. The results show that the poisoning effect is probably due to the formation of a Ti₅Si₃ coating on the surface of TiAl₃, because the Ti₅Si₃ phase has a much better crystallographic matching with TiAl₃ than it does with the Al matrix. However, TiB₂ particles appear to survive because an excessively large misfit prevents the Ti₅Si₃ phase from forming on the surface of TiB₂. The implications of this proposed mechanism are discussed in the light of current practical casting solutions and the continuing debate on the grain refinement mechanism.