Abstract: Process and apparatus for reducing emissions from internal combustion engine exhaust gas. The exhaust gas treatment system includes a first stage converter containing an NO.sub.x reduction catalyst, a second stage converter containing a catalyst for oxidation of CO and hydrocarbons, and means for admitting secondary air to the inlet of the second stage. Reduced CO and hydrocarbon emissions are obtained by admitting air in an amount not greater than that required to achieve a stoichiometric mixture to the inlet of the first stage during cold (i.e., choked) engine operation, discontinuing the flow of air to the first stage inlet when the average air-fuel ratio reaches approximately its normal warm engine value, and thereafter continuing operation without adding an appreciable quantity of air to the inlet of the first stage. The engine is operated with a substantially net rich average air-fuel ratio during warmup and with a slightly net rich average air-fuel ratio thereafter.