The role of post-combustion oxidation in influencing exhaust hydrocarbon emissions from spark ignition engines has been identified as one of the major uncertainties in hydrocarbon emissions research [l]*. While we know that post-combustion oxidation plays a significant role, the factors that control the oxidation are not well known. In order to address some of these issues a research program has been initiated at Drexel University. In preliminary studies, seven gaseous fuels: methane, ethane,ethene,propane,propene, n-butane, 1-butene and their blends were used to examine the effect of fuel structure on exhaust emissions. The results of the studies presented in an earlier paper  showed that the effect of fuel structure is manifested through its effect on the post-combustion environment and the associated oxidation process. A combination of factors like temperatures, fuel diffusion and reaction rates were used to examine and explain the exhaust hydrocarbon emission levels. However, the exhaust HC levels represent a cycle averaged result and do not identify the importance of the various factors at various stages of the expansion and exhaust. Fast flame ionization detectors have been used to identify a typical hydrocarbon signature in the exhaust port of engines [3, 4, 5 and 6]. A typical FID trace is shown in Figure 1. In the exhaust process the hydrocarbons exit the cylinder in two distinct peaks. The first peak at exhaust valve opening corresponds to the blowdown of hydrocarbon laden gases near the exhaust valve and the second peak at the end of the exhaust stroke corresponding to the expulsion of the roll up vortex. The temperatures and stoichiometries experienced by the hydrocarbons at various stages of the exhaust are significantly different and this is expected to play an important role on the extent of fuel consumption and formation of products of incomplete combustion (PIC). Time resolved species composition data of the gases coming out of the engine cylinder will be invaluable in examining such issues and in understanding the relevant in-cylinder processes. Such results are reported in this paper.