Under the auspices of the Coordinating Research Council, a program was conducted to determine the effect of gasoline/oxygenate blends on exhaust emissions (particularly carbon monoxide) for three types of emission control technologies. Tests were performed at sea level and high altitude and at temperatures of 75°F, 50°F, and 35°F. The primary fuel set consisted of a 13 psi RVP hydrocarbon-only gasoline, a hydrocarbon/11 volume percent MTBE blend, and a hydrocarbon/splash-blended 10 volume percent ethanol blend. Additionally, sea level tests were conducted with a hydrocarbon/11 volume percent ethanol matched-volatility blend and at limited conditions with a hydrocarbon/16 volume percent MTBE blend. The cars and emission control technologies tested were six 1986-88 “Adaptive Learning” closed-loop three-way catalyst systems, six 1983-86 closed-loop three-way systems without the adaptive learning capability, and four 1979-80 carbureted oxidation catalyst systems. The Adaptive Learning vehicles exhibited a small CO emissions reduction (grams/mile) at high altitude and very little response at sea level. The closed-loop, nonadaptive cars showed a variable reduction in CO with increasing fuel oxygen at high altitude and a diminished response at sea level. The oxidation catalyst cars showed the greatest reduction of CO emissions in response to fuel oxygen content over the range tested. None of these technologies necessarily responded to fuel oxygen in a linear manner.