Abstract: A catalytic promoting material is supported on a monolithic support member, of the type generally referred to as a "honeycomb" support, having longitudinal fluid flow channels extending through it. The catalytically promoting material is dispersed as a coating on the walls of the fluid flow channels which are nominally polygonal, preferably nominally square, in cross section profile so that the walls forming the flow channels can be made as thin as possible consistent with an acceptable thermal and mechanical strength. However, the juncture of the channel walls forming the flow channels are filleted to provide a concave profile, preferably one having an arcuate shape in cross section, so that the corners are "filleted" or "rounded", to subtend an appreciable segment of the nominal width of the channel wall. This prevents or at least reduces the accumulation of excess catalytic material which occurs in the sharp angular corners of prior art structures.