Abstract: This catalytically active or inactive diffuser is suitable for exhaust gas cleaning systems of internal combustion engines. It consists of a ceramic structure (8) which is obtained from a ceramized polyurethane foam. This ceramic structure (8) can be coated with surface area-increasing and/or oxygen-storing materials and/or noble metals in order to bring about a catalytic action with the gas molecules flowing through it. Because the diffuser is porous in every direction in its interior, a gas and its pressure can spread out equally on all sides through the entire diffuser volume. A virtually homogeneous velocity distribution arises over the entire flow cross section, and increased turbulence is obtained, which leads to more wall contacts of the harmful substance molecules. At the same time, the flow to and through downstream systems over their cross sections with homogeneously mixed exhaust gas including any reactants metered in is more uniform.