Natural gas appears to be the alternative fuel of choice for both light-duty and heavy-duty applications. Conversion kits are typically retrofitted to existing gasoline or diesel fueled engines. One problem area is the very limited amount of meaningful exhaust emissions test data on vehicles converted to natural gas, especially for heavy-duty vehicles. In an attempt to characterize the air quality implications of a large-scale natural gas conversion of the City of Houston's fleet of over 9,000 vehicles. a demonstration program was initiated Nine of the City's heavy-duty trucks were converted by the City to operate on compressed natural gas (CNG) fuel. The test fleet included two 1988 model year Ford F700 trucks equipped with 7.0-· gasoline engines, two 1986 model year GMC Model C7D042 trucks equipped with naturally aspirated GMC 8.2-· diesel engines, and two 1983 model year GMC Brigadier trucks equipped with 1990 model year DDC 6V-92 diesel engines that were modified by Stewart & Stevenson for dual-fuel operation. Exhaust emissions were measured from these trucks in a baseline condition and again after conversion to natural gas. The trucks were tested using a heavy-duty chassis dynamometer at 70 percent of their gross vehicle weight rating (GVWR). Emissions of total hydrocarbons (THC), non-methane hydrocarbons (NMHC), carbon monoxide (CO), oxides of nitrogen (NOx), and diesel particulate are reported in grams/mile Fuel economy is reported in energy-equivalent miles/gallon The results from this test program dismiss the widely held conception that simply converting a vehicle to run on natural gas will result in dramatic reductions in exhaust emissions. The general conclusion from these tests was that none of the natural gas conversion systems tested justified wide scale conversion to natural gas based solely on exhaust emissions performance.