The effects of operating temperature, equivalence ratio, and exhaust gas dilution on engine-out exhaust emission species has been assessed. Tests were performed on a dynamometer-mounted 4-cylinder 2.2L SI engine operating a) under steady-state, moderately cold, part-throttle conditions, and b) under conditions similar to that incurred during cold starting. Exhaust species measurements allowed the determination of specific reactivity, toxic air pollutant levels, and ozone forming potential (OFP). In addition, the unreacted fuel fraction present in the hydrocarbon emission sample was estimated.Under steady-state conditions rich equivalence ratios, low operating temperatures, and charge dilution levels beyond the misfire limit yielded low exhaust specific reactivities, but high hydrocarbon and OFP mass. Because specific reactivity increased with air dilution, OFP mass was at a minimum near the stoichiometric equivalence ratio. At the stability-limit level, dilution with exhaust gas yielded fewer toxics, lower specific reactivities, and similar net specific OFP levels relative to air dilution.Simulated cold-start tests showed that exhaust gas specific reactivity was influenced by equivalence ratio, starting temperature, and warm-up time. During starting, initial specific reactivity levels were low and increased as the engine warmed. These results suggest that faster converter light-off can reduce tailpipe reactivity as well as hydrocarbon mass.