Abstract: An apparatus and method for converting exhaust gases to innocuous products is disclosed. Exhaust gas is conveyed by a first conduit to, and passed through a first multicellular unit in which a portion of the cells are catalyzed with a first catalyst for reacting at least a portion of the NO.sub.x to ammonia, and thereafter the resulting modified mixture only is conveyed by a second conduit from the first unit and to a second multicellular unit to react the ammonia and the remaining NO.sub.x to produce innocuous products. The second conduit is connected only to the first and second units. The passage of the exhaust gas through the units results in conversion of NO.sub.x, CO, and hydrocarbons to innocuous products.