We consider mixtures of two species of spherical colloidal particles that differ in their Stokes coefficients, but are otherwise identical, in the presence of an external field. Since the particle–particle and particle–field interactions are the same for both species, they are completely mixed in the thermodynamic limit in the presence of any static field. Here, we combine Brownian dynamics and dynamic density functional theory of fluids to show that for sufficiently large differences in the Stokes coefficients of the particles (and corresponding differences in their mobilities) dynamical demixing is observed. These demixed states are transient but, under certain conditions, packing effects compromise the relaxation towards the thermodynamic states and the lifetime of the demixed phases increases significantly.