In the application of solid oxide electrolytes and membranes to the production of electricity and formation of "value added" products, respectively, the electrolyte or membrane is operated under a severe chemical potential gradient, from air to a hydrocarbon fuel/feed. By taking this functional gradient into consideration, one can design a bilayer electrolyte or membrane that utilizes more highly conductive materials if the effective interfacial P-O2 between the two layers is above a certain threshold stability limit, In this paper, we present results on the performance and stability of bilayer electrolytes and membranes, The open circuit potential (OCP) of bilayer electrolytes, with (Er2O3)(0.2)(Bi2O3)(0.3) (ESB) on the high P-O2 side and (Sm2O3)(0.1)(CeO2)(0.9) (SDC) on the low P-O2 side, was measured and the effect of relative thickness on area specific resistance (ASR) was evaluated, The oxygen permeability and stability of bilayer membranes with La0.6Sr0.4Co0.8Fe0.2O3 (LSCF) on the high P-O2 side and SDC on the low P-O2 side were measured. The results show that with this approach, higher conductivity materials can be used resulting in higher performing devices.