Al–Ti–B is the most widely used grain refiner for many Al alloys. However, the precise mechanism of grain refinement is still not clear after 60 years of intensive research. This work aims to further our understanding on the grain refining mechanism involving Al–Ti–B-based grain refiners. Extensive high-resolution electron microscopy investigation has confirmed the existence of a Ti-rich monolayer on the (0 0 0 1) TiB₂ surface, which is most likely to be a (1 1 2) Al₃Ti two-dimensional compound (2DC). Further experimental investigation was carried out to understand the potency of TiB₂ particles and the stability of the Al₃Ti 2DC. Our results showed that the potency of TiB₂ particles is significantly increased by the formation of a monolayer of Al₃Ti 2DC on their surface. The Al₃Ti 2DC forms at the liquid–Al/TiB₂ interface in concentrated Al–Ti solutions, but dissolves in dilute Al–Ti solutions, although the kinetics of both the formation and dissolution of Al₃Ti 2DC are relatively sluggish. Effective grain refinement by the Al–5Ti–1B grain refiner is directly attributed to the enhanced potency of TiB₂ particles with the Al₃Ti 2DC and sufficient free Ti solute in the melt after grain refiner addition to achieve the columnar-to-equiaxed transition.