A research program at West Virginia University sought to identify and quantify the individual hydrocarbon species present in alternative fuel exhaust. Compressed natural gas (CNG) has been one of the most widely researched fuels proposed to replace liquid petroleum fuels. Regulated CNG non-methane hydrocarbon emissions are often lower than hydrocarbon emissions from conventional liquid fuels because of the absence of heavier hydrocarbons in the fuel. Reducing NOx and non-methane organic gas (NMOG) emission levels reduces the ozone forming potential (OFP) of the exhaust gases. A Hercules GTA 3.7 liter medium duty CNG engine was operated at seven load and speed set points using local supply CNG gas. The engine was operated at several rated, intermediate and idle speed set points. The engine was operated while the air/fuel ratio value was varied. The lighter load tests as well as the lean operation tests showed an increase in the hydrocarbon emissions as a result of unburnt fuel passing through the engine. As the lambda value approached stochiometric operation, NOx increased and the HC decreased. Hydrocarbons were captured using a dedicated sampling system drawing a sample from a full scale dilution tunnel via a rotary vane pump through a mass flow controller and collected in a tedlar bag. The sample was then analyzed by a gas chromatograph for identification and species were quantified utilizing retention indices. Upon close analysis of the data, the production and destruction of compounds present in engine exhaust can be related to the test fuel and background dilution air. The intent of this research was to identify the formation of products such as ethene that arise from incomplete combustion.