Exhaust gas sampling was used to understand how spark-ignited internal combustion enginesemit unburned hydrocarbons(HCs) and form non-fuel HCs. A sampling unit collects exhaust gasduring a given 600 microsecond interval of the cycle, from a specific location within the port. Gas chromatography is used to measure individual HC species concentrations; total HC concentrations are in agreement with those measured by a fast flame ionization detector mounted in parallel with the sampling unit. Spatial resolution of port concentrations shows that the gas is well mixed a short distance from the valve, as no radial gradients are apparent. Time resolved measurements during the cycle reveal the distribution of species during the exhaust period, while measurements along the exhaust port axis mark the progress of reaction of each species. The results show that a higher fraction of non-fuel HCs leaves the cylinder during the late period of the exhaust phase, and that a significant fraction of HCs are partially or completely oxidized in transit along the port axis.