Apparatus for controlling emissions from an internal combustion engine including an enclosed housing one end of which is provided with an inlet and the other end an outlet. A coaxial tubular member extends from the inlet into the housing. A catalytic cell surrounds the tubular member and separates two chambers, a first surrounding the tubular member near the inlet and the other on the opposite side of the catalytic cell. Exhaust gases from the internal combustion engine flow through the tubular member, expanding into the other chamber, then through the catalytic cell for conversion of nitrogen oxides, carbon monoxide and unburned hydrocarbons to less noxious components, expanding into the first chamber for eventual discharge through the outlet.