A group of GM C-2500 heavy-duty and GM C-1500 light-duty trucks were modified to bi-fuel compressed natural gas (CNG)/gasoline and liquefied petroleum gas (LPG)/gasoline vehicles. The fuel management systems used for different model year vehicles were introduced. The Alternative Fuel Technology (AFT) System used for 1997 GM C-2500 trucks is discussed in detail. AFT is an automatic switching bi-fuel system which is able to control fuel flow rate, spark timing, and EGR, and perform On-Board Diagnostics (OBD-II). The exhaust emissions of carbon monoxide (CO), total hydrocarbon (HC) and oxides of nitrogen (NOx) were measured using dilute sampling techniques. The vehicles tested were operated on a chassis dynamometer and run on the Federal Test Procedure (FTP) Urban Schedule.It was found that compared to those from baseline gasoline vehicles, the NOx emissions from both CNG and LPG vehicles were significantly reduced, and that the HC and CO emissions from LPG vehicles were significantly reduced. However, the total HC emissions from CNG vehicles were substantially higher than those from the baseline gasoline vehicles. The CO emissions of the CNG vehicles showed different trends between different model year vehicles using different fuel management systems. For the 1997 C-2500 trucks using the AFT system, the CO emissions were lower than the baseline gasoline vehicles. The fuel economy on a gasoline equivalent energy basis of CNG and LPG vehicles were about 22% and 4% lower respectively, than that of the baseline gasoline vehicles.