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Real time individual hydrocarbon tailpipe emission monitoring on a ‘lambda one’ (stoichiometric air/fuel ratio, i.e. λ = 1) controlled vehicle, driven in the ‘Neuer Europäischer Fahrzyklus’ (NEFZ) driving cycle on the chassis dynamometer, showed dramatic variations in the concentrations of butadiene and benzene when operated with gasoline having different octane ratings. Butadiene and benzene emissions were higher in 91 octane (RON) fuels compared to 95 and 98 octane fuels. The aromatics content of RON 91 fuel was significantly lower and the concentration of benzene was equal to the RON 95 fuel. The measured emissions of total hydrocarbons (THC) cannot reflect these differences, whereas cold start behaviors as well as highway driving emissions are derived from the test program.