Powerful catalysts may be produced from gas atomized Al-Ni powders. The efficiency of these catalysts depends on the amount of the different phases formed during solidification. The present paper reports on the analysis of these transformations by combining experiments and simulations. Firstly, the volume fractions of phases in gas atomized Al-Ni powders are determined for several compositions and size ranges using neutron diffraction analysis. Additionally, solidification paths are simulated with a model previously validated for concurrent dendritic, peritectic and eutectic phase transformations in binary alloys. The measurements demonstrate a strong dependence of phase fractions on particle size, as well as an inversion of trends with composition. The predictions of the model show good agreement with the measurements. Details of the simulations clarify the combined roles of the kinetics of cooling, the solute diffusion in phases as well as the growth kinetics of microstructures. These advanced interpretations should open the way to the production of further optimized catalysts.