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The “co-poisoning” effect between Zr and Ti (derived from Al-Zr and Al-Ti-B master alloy additions) on the grain refinement of cast aluminum alloys is studied from a crystallographic atom matching viewpoint. The edge-to-edge matching (E2EM) model has been used to investigate the possible “poisoning” phase containing Zr/Ti, Al, and Fe in commercial grade aluminum alloys. The results show that Al₃Ti is the most likely constituent to be poisoned due to the formation of an Al₈Fe₄Zr coating on its surface, since the Al₈Fe₄Zr phase has good crystallographic atom matching with Al₃Ti, but not with the aluminum matrix. Meanwhile, the partial dissolution of Al₃Zr nucleant particles to compensate for the loss of solute Zr aggravates the poisoning phenomenon. This proposed mechanism is consistent with most previous experimental observations and with existing practical solutions employed in the foundry.