Natural gas is a viable alternative to gasoline and diesel fuel because it is a clean burning fuel that is available from a large domestic reserve through a mature infrastructure. The heavy dependence of the small engine sector on oil, much of which is imported from foreign countries and the small engine sector's negative impact on the air quality in urban areas are two pervasive problems that can be helped by using Compressed Natural Gas (CNG) as a small engine fuel. In addition, CNG is typically over 80% methane, which is produced by the decay of organic material, so while natural gas is not renewable its use enables much of the infrastructure required for a methane-based renewable energy system.In order to determine the emissions benefit of using CNG as compared to gasoline in a small engine, a 750 cc 90 degree V Twin port-fuel-injected production engine rated at 29 horsepower (HP), designed and built by Kohler Inc. This engine was initially fueled by gasoline, but was converted to a port-fuel-injected CNG engine. The engine emissions measured include total hydrocarbons (HC), carbon monoxide (CO), carbon dioxide (CO₂), oxides of nitrogen (NOx), oxygen (O₂), and intake CO₂, using both diluted and raw exhaust gas emissions. This paper will briefly describe the modifications made to the engine to operate on CNG (fuel injection system, engine calibration...), the primary focus, however, will be the comparison of the emissions data obtained for both the original engine configuration, gasoline-fueled, and the modified configuration, CNG-fueled. While this paper will focus on the emissions benefit of using CNG and CNG's effect on engine performance parameters, this paper will also review the sensitivity of the engine to changes to several engine parameters and the resulting engine emissions.