Development of methodology for diesel hydrocarbon speciation of C12-C22 compounds and the application of that methodology to determine total ozone forming potential of diesel exhaust emissions is an extremely complicated task. Methodology has already been developed for speciating C1-C12 exhaust emissions from engines and vehicles fueled with gasoline, diesel, and alternate fuels. However, very little or no information is available for exhaust speciation of C12-C22 compounds as sampling and analytical constraints make the collection and analysis of the higher molecular weight compounds extremely challenging. Key issues related to the definition of “hydrocarbons” also need to be addressed prior to promulgation of future reactivity-based legislation for diesels (e.g., Which exhaust hydrocarbon compounds actually exist in gas-phase and participate in atmospheric ozone formation?). In an attempt to begin to address these issues, preliminary sampling and analytical methodology were developed and tested utilizing a Cummins C Series Model C8.3-250 diesel engine operating over a heavy-duty FTP cold-start emission test cycle. This methodology shows promise for use in effectively collecting and analyzing C12-C22 hydrocarbons from transient emission tests.