The microscopic appearance of eutectic Ai-Si alloys depends on their cooling rate. Slow cooling produces a coarse appearance, whereas at high speeds a very fine biphasic mixute containing solid alpha dendritic crystals is formed. The approximately 18 % Si concentration of this mixture is distinctly higher than the equilibrium eutectic (11.7 %). The possibility that this difference is due to the fact that Al is four times more thermally conductive than Si, while its fusion heat is over four times lower, is discussed. The addition of small amounts of Na, K or Sr leads to metallographic structures fully analogous with those of the rapidly cooled alloys. These agents employ different mecanisms to destroy the Si crystallization seeds, and so impede their nucleation. This results in subcooling of the liquid equivalent to that caused by sudden cooling.