As more stringent vehicle emission standards are introduced worldwide, there is an increased need to provide a thorough assessment of the environmental impact of alternative fuels. With the advent of CNG as a viable transportation fuel, the development of advanced computer controlled fuel delivery systems is imperative in order to ensure acceptable emission performance. At present, the majority of light and medium duty engines operating on natural gas are primarily gasoline automotive engines which have been retrofitted to allow for the use of CNG.The Mobile Sources Emissions Division of Environment Canada and the Canadian Gas Association have conducted a joint test program in order to develop a database of exhaust emissions from vehicles typically converted for operation on either gasoline or natural gas at various operating temperatures. The four light duty trucks selected for the program represented current commercially available natural gas fuel metering technologies designed for bi-fuel mode applications utilizing computer controlled electronic feedback fuel injection. Gaseous tailpipe exhaust measurements of carbon monoxide, oxides of nitrogen, carbon dioxide, methane, non-methane hydrocarbons, and aldehyde/ketones were determined over the Urban Dynamometer Driving Schedule at 24°C and -18°C at 10,000 km, odometer reading and again at 24°C at 50,000 kilometers. In addition to providing emissions data for the systems the test program also examined the effects of temperature and mileage accumulation on the test vehicle emissions.