A new phase selection model based on the time-dependent nucleation theory was developed to investigate the effect of rapid solidification on extended solubility. The model was applied to predict the solubility as a function of undercooling for several binary Al alloys. The predictions of both eutectic and peritectic systems show good agreement with experimental data. It was demonstrated that the developed model is better than the T 0-line method, which neglected the kinetic process of nucleation. Furthermore, the model can also be applied to ternary and multicomponent phases assuming the nucleation is limited by the scarcest species or the slowest diffuser. The feasibility and reliability of the new model make it a useful tool for novel alloy design for rapid solidification processes such as additive manufacturing.