Mixed-conducting oxides have been used in many applications, including fuel cells, gas-separation membranes, sensors, and electrocatalysis. We are developing a mixed-conducting, dense ceramic membrane for selectively transporting oxygen and hydrogen. Ceramic membranes made of Sr-Fe-Co oxide, which has high combined electronic and oxygen ionic conductions, can be used to selectively transport oxygen during the partial oxidation of methane to synthesis gas (syngas, CO + H-2). We have measured the steady-state oxygen permeability of SrFeCo0.5Ox as a function of oxygen partial-pressure gradient and temperature. At 900 degrees C, oxygen permeability was approximate to 2.5 sec cm(-2) min(-1) for a 2.9 mm thick membrane and this value increases as membrane thickness decreases. We have fabricated tubular SrFeCo0.5Ox membranes and operated them at 900 degrees C for > 1000 h during conversion of methane into syngas. The hydrogen ion (proton) transport properties of Ba(Ce,Y)O3-delta were investigated by impedance spectroscopy and open-cell voltage measurements. High proton conductivity and a high protonic transference number make Ba(Ce,Y)O3-delta a potential membrane for hydrogen separation.