A solid state membrane for a reforming reactor is disclosed which comprises at least one oxygen anion-conducting oxide selected from the group consisting of hexaaluminates, cerates, perovskites, and other mixed metal oxides that are able to adsorb and dissociate molecular oxygen. The membrane adsorbs and dissociates molecular oxygen into highly active atomic oxygen and allows oxygen anions to diffuse through the membrane, to provide high local concentration of oxygen to deter formation and deposition of carbon on reformer walls. Embodiments of the membrane also have catalytic activity for reforming a hydrocarbon fuel to synthesis gas. Also disclosed are a reformer having an inner wall containing the new membrane, and a process of reforming a hydrocarbon feed, such as a high sulfur-containing diesel fuel, to produce synthesis gas, suitable for use in fuel cells.