Abstract: An improved catalytic reactor for high temperature reactions having a reaction chamber and a monolithic catalyst structure disposed in the reaction chamber is disclosed wherein the catalyst structure has a multiplicity of longitudinally disposed channels formed by thin metal substrate walls which expand on exposure to the heat generated in high temperature reactions and reactor also includes a monolithic open cellular support structure disposed in the reaction chamber having a multiplicity of longitudinally disposed passageways formed by strips of high temperature resistant metal or ceramic material with the support structure being secured on its outer periphery to the wall of the reaction chamber to limit movement along the longitudinal axis, and being positioned at and abutting against the outlet end of the catalyst structure.