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Gas chromatography tests have been applied to the exhaust gases of a spark ignition engine to determine the concentration of unburned fuel among the total hydrocarbons. The contribution of the unburned fuel was determined for variation of several engine parameters. The fuel tested was isooctane. The varied parameters were air-fuel ratio, engine speed, ignition timing, compression ratio, and coolant and lubricant temperature. The results have shown that the unburned fuel is responsible for most of the HC emitted, participating with 50 to 73%, depending on the engine working conditions. Methane, acetylene, ethylene, ethane, propylene, and isobutylene were also analyzed, apart from isooctane. The total contribution of the lighter species remained practically unaltered when the parameters were varied, though their individual concentrations did change. Thus, the unburned HC was seen to determine all trends of exhaust hydrocarbons.