The recent tightening of emission standards for new heavy duty engines has lead to the development and implementation of alternative fuel engines, particularly for urban transit bus applications. Alternative fuels are intended to offer a potential emissions benefit with regards to the regulated emissions, and especially the particulate matter, which has received the greatest degree of regulatory action. However, the entire composition of the engine emissions should be considered when evaluating the environmental benefits of these new fuels, and also the continued performance of these engines in actual fleet service. In this study the exhaust emissions from methanol, ethanol, and diesel - powered buses were determined during transient operation of the vehicles on a heavy duty chassis dynamometer. The tests of the alcohol fuelled buses, and a control diesel bus were conducted as the buses accumulated mileage in revenue generating service. The buses were tested at various simulated vehicle loadings over four driving cycles. The cycles include the EPA Heavy Duty Chassis Transient, the Central Business District, the New York Bus, and the New York Bus Composite. The exhaust emission rates of hydrocarbons, carbon monoxide, nitrogen oxides, aldehydes, particulate matter, and alcohols were determined.