The new California LEV-II regulations for “near zero” evaporative emissions require a 75% reduction from current emission levels for light duty vehicles. To meet the challenge of satisfying these new regulations, there is an immediate need for an increased understanding of the sources of evaporative emissions. Hydrocarbon speciation by gas chromatography is a powerful analytical tool for determining the composition of complex hydrocarbon mixtures. Gas chromatography and gas chromatography/mass spectrometry were used to identify the volatile organic compounds (VOC) present in the evaporative emissions from a number of prototype and recent production model gasoline-fueled vehicles. For a “typical” evaporative emissions sample, more than 90% of the emissions were found to be fuel-type hydrocarbons. When emission profiles from 2-day diurnal (SHED) tests on different vehicles were compared in terms of their gross features, discernable patterns emerged which provide valuable information regarding the sources of emissions. These profiles can be used to identify the probable cause of a failed SHED test. This report describes the application of the speciation methodology, provides examples of information obtained, and establishes a case for collecting speciation samples from all developmental SHED tests.