A catalyst substrate support is provided for a corrugated foil honeycomb matrix defining a plurality of passages extending therethrough which are generally parallel to an axis. A peripheral mantle extends about an outer perimeter of the matrix and has inwardly extending flanges which extend across an outer periphery of the opposite end faces to cover outermost of the passages and restrict fluid flow between the peripheral mantle and the matrix. The outer perimeter of the matrix and the peripheral mantle may be spaced apart to define a gap for accommodating differential thermal expansions of the matrix and the peripheral mantle, the gap being smaller than a height of the inwardly extending flanges. Cross members secured to each of the opposite end faces of the matrix may transfer at least part of the gravitational load of the matrix to the mantle.