The present invention relates to composite materials for membrane reactors which include a gas-tight ceramic, a porous support, and an interfacial zone therebetween. More particularly, this invention relates to composite materials using oxygen ion-conducting dense ceramic membranes formed on a porous support comprising a metallic alloy to provide an interfacial zone identifiable by a gradient of composition in at least one metallic element across the interfacial zone between the dense ceramic membrane and the porous support. Processes using composite materials in accordance with the invention are, for example, used for production of synthesis gas comprising carbon monoxide and molecular hydrogen which synthesis gas is, advantageously, free of deleterious and/or inert gaseous diluents such as nitrogen.