This paper reports on rapid solidification of Al-Cu alloys. A heterogeneous nucleation/growth model coupled with a thermal model of a falling droplet through a stagnant gas was developed. The primary undercooling as well as the number of nucleation points was compared with Al-Cu alloy droplets produced by Impulse Atomization (IA). Based on experimental results from Neutron Diffraction, secondary (eutectic) phases were obtained. Then, primary and secondary undercoolings were estimated using the metastable extensions of solidus and liquidus lines calculated by Thermo-Calc. Moreover, Synchrotron X-ray micro-tomography has been performed on Al-4.5wt%Cu droplets. The undercoolings are in good agreement. Results also evidence the presence of one nucleation point and are in agreement with the experimental observations. 1. Introduction Manufacturing of most metallic alloy products involves solidification at some stage. Mechanical properties of these products are generally related to their solidification microstructures. Depending on the final application of a product, a certain type of microstructure is more appropriate compared to another. For a product that requires directional properties, a microstructure of columnar grains is needed while isotropic properties are satisfied with an equiaxed structure. Generally, post-processing of the solidified materials is required to obtain the final product with desired properties. These post-solidification treatments are generally time-consuming and therefore increase the production cost without fully eliminating solidification related defects such as segregation. Therefore, it is important to understand all the dynamics involved in the formation of solidification microstructures in order to control the properties of the final products. As dendrites growth from an undercooled melt depends a great deal on the nucleation undercooling. Therefore, determination of undercooling and the resulting growth rate, recalescence, microsegregation/phase fraction and grain size is very important. Al-Cu alloys (4.5, 5, 10 and 17 wt% Cu) have been produced by IA and the last three compositions were analysed in our previous papers [1, 2]. IA is a single fluid atomization technique that is capable of producing droplets of controlled size having a relatively narrow distribution and a predictable cooling rate. The alloys (350 to 450g) were melted in a graphite crucible by means of an induction furnace and atomized at 850ºC in an almost oxygen free chamber (10ppm) under Nitrogen, Helium or Argon atmospheres. The atomized droplets rapidly solidify during their fall by losing heat to the