Small, gasoline fueled spark ignition engines are generally designed to operate at air to fuel ratios richer than stoichiometric. Consequently, they tend to emit high levels of carbon monoxide and hydrocarbons in their exhaust. This paper deals with an investigation of reducing emissions of carbon monoxide and unburned hydrocarbons by utilizing a small, metal matrix catalyst in conjunction with a thermal reactor. The experimental work was carried out on a small, single cylinder, air cooled, four stroke, spark ignition engine. The work was divided into two phases: Phase I was aimed at determining the extent to which oxidation of carbon monoxide and unburned hydrocarbons could be achieved using a two-way catalyst in conjunction with a thermal reactor. The work was later expanded to include a three-way catalyst in lieu of a two-way catalyst. In this phase controlled amounts of air from laboratory supply was used to achieve emission control. In the second phase, air valves actuated by exhaust pressure pulses were used to induct the needed air into engine exhaust.The study showed that a two-way catalyst and a thermal reactor system was able to oxidize over 95% of carbon monoxide and hydrocarbons with controlled amounts of exhaust air. The three-way catalyst system provided slightly lower performance but still in the 90% range for the three species. The air valve system was able to achieve high conversion of hydrocarbons but carbon monoxide conversion efficiencies were lower than those with controlled air, particularly at very light loads. Conversion of oxides of nitrogen were also lower than those were achieved with controlled air.Furthermore, performance of the air valve system in lowering emissions from a catalyst-reactor combination was found to depend on engine speed since operation of these valves are sensitive to engine load and speed.