Several areas of the country have mandated or are considering the use of oxygenated fuel blends for the control of wintertime carbon monoxide (CO) emissions. Some of these areas are at high altitude. The purpose of this work was to evaluate the effect of oxygenated fuels on CO emissions in modern, closed-loop vehicles and characterize the effects of altitude and temperature on CO emissions. Eight vehicles equipped with closed-loop emission control systems were tested with two oxygenated fuel blends and a base fuel (Indolene). All had Reid Yapor pressure adjusted to 10 psi. The oxygenated blends, at 3.5% (by mass) oxygen, were 10% ethanol in Indolene and 20% methyl tertiary-butyl ether (MTBE) in Indolene. Federal Test Procedure (FTP) exhaust emissions were measured with all eight vehicles at standard conditions of 74°F and 982 ft. altitude. In addition, one Throttle Body Injection (TBI) vehicle and one Port Fuel Injection (PFI) vehicle were tested with 10% ethanol at high (4000 ft.) and low (1200 ft.) altitudes, at 74°F and 40°F. The use of oxygenated blends in current-production closed-loop vehicles, decreases CO emissions. The reduction was less with the Port Fuel Injection system than with the other fuel systems tested. The effect of temperature on CO emissions was more critical than the effect of altitude.