An axial/radial—or radial/flow catalytic reactor has inlet and outlet ports and a bed of particulate catalyst disposed as e.g. a cylinder or cone round a central region communicating with one of the ports. For at least part of the height of the catalyst bed, the exterior surface of the catalyst bed has a diameter less than that of the reactor thus leaving space between the exterior surface of the catalyst bed and the interior walls of the reactor. This space is filled with a particulate material presenting less resistance to flow than the catalyst particles.