Abstract: An internal combustion engine exhaust gas catalytic converter has coaxial inner and outer louvered tubes which define an annular catalyst bed. An outer shell coaxial with the inner and outer louvered tubes define a portion of a gas flow path downstream from the catalytic bed. The downstream flow portion has an annular outlet of smaller cross sectional area than the cross-sectional area of the inlet end of an upstream portion of the flow path. Heat expandable means maintain a constant density of catalyst beads within the catalytic bed. These heat expandable means preferably comprise a composition of ceramic fibers, vermiculite and a binder.