Species analyses have been performed on engine-out and tailpipe hydrocarbon mass emissions to help understand why fuels with higher T50 and/or T90 distillation temperatures produce higher engine-out and tailpipe hydrocarbon emissions and why fuels with higher T90 distillation temperatures produce higher engine-out and tailpipe specific reactivities. Species analyses were also performed to examine the effects of fuel sulfur level on engine-out and tailpipe species and specific reactivities. These analyses were performed on three different test-vehicle fleets representing varying levels of emissions control technology and the effect of emissions control technology was examined.Individual hydrocarbon species concentrations in both the engine-out and tailpipe were found to correlate linearly with the concentrations of the same species in the fuel, implying that a small fraction of the fuel escapes the combustion process and conversion over the catalyst. Analysis showed that approximately half of both the engine-out and tailpipe hydrocarbons consists of unreacted fuel. Engine-out, tailpipe, and fuel specific reactivities all correlated with one another and all decreased with decreasing fuel T90. However, the decrease in specific reactivity was not due directly to T90 but due to the replacement of highly reactive, high boiling point dialkyl and trialkyl aromatics with lower reactivity, low boiling point aromatics which was required to achieve the lower fuel T90. The level of emission control technology had only minor effects on the observed species and specific reactivity correlations.Decreasing fuel sulfur had no effect on engine-out hydrocarbon mass or species but decreased tailpipe hydrocarbon mass by decreasing the conversion inefficiencies of all hydrocarbon species. While all species conversion inefficiencies decreased, the inefficiencies of the paraffins decreased to a greater extent increasing the relative tailpipe concentrations of the more reactive olefins and aromatics, thereby increasing tailpipe specific reactivity as fuel sulfur level was decreased.