The solid oxide membrane (SOM) process has been used to produce magnesium by direct electrolysis of its oxide. In this process, MgO is dissolved in a molten CaF2–MgF2 flux and an yttria-stabilized zirconia (YSZ) SOM membrane separates the cathode and the flux from the anode. YSZ membrane stability limits the operating life of the SOM process. This study investigates the interactions between YSZ and the molten CaF2–MgF2 salt. The YSZ membrane is known to degrade due to diffusion of yttria into the flux. Yttria diffusion can, however, be decreased or prevented by adding YF3 to the flux. When the activity of yttria in the flux is greater than that of yttria in the membrane, yttria is observed to diffuse from the flux into the YSZ. Yttrium concentration profiles in the YSZ membrane as a function of time and YF3 content in the flux were determined using energy dispersive X-ray spectroscopy (EDS). An analytic solution to the diffusion equation was used to model the diffusion process. This study helps to determine the optimum YF3 content needed in the flux to minimize yttria diffusion and increase membrane stability.