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Under any driving conditions, the gas flow in present-day monoliths for automotive exhaust catalysts shows laminar characteristics resulting in thick boundary layers. These tend to restrict mass transfer, a highly desirable prerequisite to good catalytic activity. Based on the theory of catalysis and in the light of such all-important mass transfer, the design of the existing type of catalyst metal supports has been revised. The new metal supports vary from their predecessors by largely split and slotted honeycomb passages. In this way, gas flow is split and divided over and over again. This results in thin boundary layers and turbulence on the catalytically active support surfaces, with intense mass transfer between the core stream and the surface-contacting zone. The resultant improvements are discussed in terms of catalytic effect, size, weight, cost, internal flow, temperature equalization and component strength.