The contribution of non-ideal mixing for the crystallization of supercooled mixtures of argon and krypton is reported, showing that this process is well described by classical crystal growth theories when such thermodynamics is considered. Crystallization is a fundamental process in materials science, providing the primary route for the realization of a wide range of new materials. Crystallization rates are also considered to be useful probes of glass-forming ability(1-3). At the microscopic level, crystallization is described by the classical crystal nucleation and growth theories(4,5), yet in general solid formation is a far more complex process. In particular, the observation of apparently different crystal growth regimes in many binary liquid mixtures greatly challenges our understanding of crystallization(1,6-12). Here, we study by experiments, theory and computer simulations the crystallization of supercooled mixtures of argon and krypton, showing that crystal growth rates in these systems can be reconciled with existing crystal growth models only by explicitly accounting for the non-ideality of the mixtures. Our results highlight the importance of thermodynamic aspects in describing the crystal growth kinetics, providing a substantial step towards a more sophisticated theory of crystal growth.