a ceramic membrane structure and method for separating oxygen from an oxygen containing feed at temperatures above about 600 ° c . the membrane is provided with a dense layer and one or more active porous layers . the dense layer contains at least a mixed conducting material and the active porous layer is formed of a mixture having an ion conducting phase capable of predominantly conducting oxygen ions and a mixed conducting phase capable of conducting both oxygen ions and electrons . the ion conducting phase is present within the mixture in an amount greater than a percolation threshold and the mixed conducting material and phase have a greater electronic conductivity than ionic conductivity .