An Ellingham diagram is useful to both synthesis and application of ceramic membrane material. Dense ceramic oxygen-transporting membranes rely on mixed ionic-electronic conductors (MIECs), which are complex oxides with an oxygen non-stoichiometry. Often these oxides have more than three different cations incorporated. Alkaline-earth metals among the cations make the ceramic membrane potentially vulnerable to the formation of oxygen impermeable carbonate scales on its surface, if the membrane is operated in CO2-containing atmosphere. Here it is shown that an Ellingham diagram, which includes not only the temperature-dependent equilibrium carbon dioxide chemical potential over a mixture of a metal’s carbonate and its oxide, but also of the surrounding atmosphere, allows identify save operational conditions for a given MIEC. Moreover, the Ellingham diagram allows identify cations to build a MIEC for specific operational conditions. For synthesis of MIECs, minimum applicable calcination temperature, to decompose carbonate intermediates, can be read from the Ellingham diagram.