Fuel-exhaust compositional relationships are derived for exhaust hydrocarbons and aldehydes using data obtained with 18 full-boiling-range gasolines used as fuel in a late model vehicle. The agreement found between the data from the present work and those obtained by others in similar studies indicates that the relationships should be sufficiently general to apply to a large segment of the current vehicle population. The present data indicate that aromatic hydrocarbon and aromatic aldehyde emissions are linearly related to fuel aromatic content, while exhaust olefin and aliphatic aldehyde emissions show an inverse relationship. Regression analysis of the aromatic hydrocarbon and aromatic aldehyde data, expressed as a function of fuel aromaticity, gives fuel aromatic coefficients of 0.49 and 0.41 for the hydrocarbon and aldehyde expressions, respectively. Exhaust photochemical reactivity calculations, using a composite reactivity scale, predict that fuel aromatic content has little influence on exhaust reactivity. The fuel-exhaust compositional relationships are shown to be insensitive to the nature of the driving cycle. A comparison of data obtained using the 1968, 1972, and 1975 federal test procedures show the existence of only minor differences in exhaust composition. However, elimination of the cold start from the 1972 test procedure did have a significant effect on the results.