In this study, we investigated the crystallization of a salt mixture during drying in a porous medium. Specifically, we focused on the ternary system of NaCl–NaNO3–H2O, which is also encountered in situ. In order to study the crystallization, we use a specialized 4.7 T NMR setup that allows us to directly measure the NaCl and NaNO3 concentrations in a porous medium and track their ratio during drying, providing direct insight into the phase diagram. The measurements indicate that the equilibrium phase diagram alone is not sufficient to describe the physical processes that occur in porous media during drying experiments. In the case of forced drying in this study, where advection of the ions is dominant (Pe > 5), the measurements indicate that we need to take supersaturation into account and that crystallization is driven by transport. As a result, the ratio of a salt mixture will remain constant in the porous medium throughout the experiments, as was seen for this ternary system Na+, Cl–, NO3– resulting in the formation of both NaCl and NaNO3. These results indicate that the rate of evaporation, in combination with the effect of supersaturation and solution transport in the pore system, allows the saturation degree given by the phase diagram to be surpassed. This phenomenon is critical when assessing mixed salt systems in porous media and should be considered when evaluating phase diagrams alone.