The heavier components of a gasoline appear to contribute disproportionately to hydrocarbon emissions. To quantify this effect, a series of well defined bi-component fuels consisting of a mid-range volatility base (isooctane) with a variety of heavy hydrocarbons have been tested in a single cylinder test engine under steady conditions. The fraction of the heavy component in the fuel that was emitted in the exhaust was greater than that predicted from the correlation that has been developed with its OH reactivity. The extent of the discrepancy increased with the concentration of heavy component in the fuel or with the boiling point of the heavier hydrocarbon. The results are consistent with a mechanism based on an increased probability of wall impaction of heavy components during the intake stroke because of their slowness to evaporate. The impacted hydrocarbon has an increased probability of being collected in the piston-top crevice during compression and so being protected from the main combustion. A simple theory based on this mechanism has been developed and can semi-quantitatively predict the dependence on the volatility and concentration of the heavy fuel component.