A single-cylinder, 4-stroke, spark-ignition engine was used to evaluate the effect of oxygen enriched air on engine performance and exhaust emissions. Evaluations were made with both gasoline and natural gas. The oxygen content of the intake air was varied between 20.9% (ambient air) and 25%. The effects of oxygen enrichment were evaluated in terms of power output, specific fuel consumption, fuel conversion efficiency, exhaust gas temperature, and exhaust emissions (carbon monoxide and hydrocarbons). Test results indicate that the use of oxygen enriched air produces a significant increase in power output, improved fuel conversion efficiency, lower specific fuel consumption, higher exhaust gas temperature and a substantial reduction in carbon monoxide and hydrocarbon emissions when the engine is fueled with either gasoline or natural gas.The feasibility of using a membrane gas separator to supply oxygen enriched air for vehicle applications was also considered and determined to be feasible. Additional research is needed to fully explore the possibilities of reducing exhaust emissions and increasing engine efficiency via oxygen enriched intake air.