Abstract: A catalytic reactor of oval cross section comprises a generally cylindrical catalytic substrate clamped between paired housing shells and spaced therefrom by a mesh support that extends around the oval periphery of the substrate and slightly overlaps the latter's axially opposite ends to provide locating projections. The support and housing are preferably interlocked by one or more pairs of inwardly opening channels of the housing having portions of the mesh support confined therein, each pair being spaced axially by a rib of the housing projecting into the mesh support. The housing has axially endwise converging cam portions adjacent and axially endwise of the opposite ends of the substrate to engage the locating projections and center the assembled substrate and mesh support within the housing during manufacture of the reactor.