In pure aluminum the nucleation of a new solid phase during the liquid-to¬solid phase transformation is activated by a relatively large undercooling of the melt with respect to the transformation temperature. The requirement of this undercooling to activate nucleation manifests itself in a large gram size and an inhomogeneous gram morphology, in the solidified material. However, when a solid surface compatible with the lattice of the forming nucleus is present in the melt, less undercooling is needed to activate nucleation. Therefore generally a small fraction of microscopic TiB2 particles (grain refiners) are added to aluminum alloys before solidification. It appears that without the addition of a small amount of extra solute titanium in the melt, the surface of these particles does not work as an effective nucleation site. Even then, only about 1 % of the particles added to the melt nucleate grains during solidification. The remainder are of no inherent value to the final product. Contrary to that, agglomeration of these particles, can results in casting defects in the solidified material. This study provides a new insight into the nature of gram nucleation and growth during solidification of gram refined aluminum alloys and relate the microstructural features to the gram refiner efficiency.