On treating aluminium-silicon alloy with 0.2 wt% Sb, it was revealed that antimony refines the eutectic structure by reducing the interflake spacing rather than acting as a modifier. The growth mechanism is similar to the unmodified Al-Si flake structure, giving the relationships of the type DELTAT=K1V0.51 and lambda=K2V-0.4, where K1 and K2 are constants. At high solidification rate, the transition from flake to fibre is observed. However, this transition occurs at lower velocity compared to quench modification of the pure alloy. The high magnitude of undercooling measured with the antimony-treated alloy is attributed to constitutional undercooling, which leads to extra refinement of the Al-Si eutectic structure.