This paper discusses five vehicle emissions testing programs conducted by the Colorado Department of Health (CDH) to evaluate “gasohol” (a mixture of 10% ethanol and 90% gasoline, by volume) as a potential CO reduction strategy for high altitude locations. These testing programs included: Forty-seven 1978-1983 passenger cars. Five 1982 “high mileage” passenger cars. Two passenger cars (one 1984 and one 1985) with reported “adaptive learning” capabilites. Eleven 1979-1984 passenger cars tested at sub-FTP temperatures. Eight non-catalyst light and heavy-duty trucks. Mass emissions levels of hydrocarbon, carbon monoxide and oxides of nitrogen were measured during these test programs. In addition, the forty-seven vehicle study also includes fuel economy and driveability comparisons when using gasoline and gasohol.