A numerical model is developed describing the very fast process of microstructure evolution under the common action of nucleation and diffusional growth of minority phase droplets during cooling of a hypermonotectic alloy through the miscibility gap. The model is validated comparing it with an analytical solution obtained with some simplifying assumptions and then applied to predict the microstructure evolution in Al-Pb alloy in a real casting situation. The numerical model reflects the real physical processes well and also yields some general relations applicable to other immiscible alloys.