For grain refinement of aluminium and its alloys, a new aluminium-based master alloy containing TiC nucleants was developed a few years ago, by reacting carbon with Al-Ti binary alloy melt. However, as the melt was held at the usual melting temperatures of below 1273 K, the resultant master alloy lost its grain-refining efficiency, and so this phenomenon was called the "poisoning" effect. Nevertheless, a superheating treatment of the melt at higher temperatures (>1523 K) rejuvenated the nucleant particles. The present investigation, dealing with electron diffraction of carbide particles extracted from the poisoned master alloy, revealed massive formation of Al4C3 and Ti3AlC, of which Al4C3 appears to account for the poisoning of TiC nucleants. On the other hand, subsequent electron diffraction studies on the rejuvenated nucleants confirmed that they were essentially composed of uncontaminated TiC.