Data obtained from six vehicles equipped with current and advanced emission control systems are used to define fuel-exhaust compositional relationships for hydrocarbons and aldehydes, with special emphasis on photochemically reactive exhaust species. The vehicle emission control system is shown to strongly influence hydrocarbon mass emission level and hydrocarbon composition. However, the distribution of individual species within the two reactive hydrocarbon classes, olefins and aromatics, is shown to be only slightly affected by vehicle system. The exhaust olefin data from the current systems are used to define the relationship between exhaust olefin composition and branched character of the fuel paraffin fraction. Branched fuel paraffins are shown to favor propylene and C4 olefin production while straight chain paraffins are shown to favor ethylene production. Regression expressions are presented which relate individual and subclass exhaust olefin concentrations to the concentrations of various hydrocarbon classes in the fuel. The concentration of C7 and higher exhaust aromatic components are shown to be adequately described in terms of the concentrations of the corresponding components in the fuel. Correlations between exhaust benzaldehyde content and fuel toluene content, as well as between exhaust tolualdehyde and fuel xylene content are given, which appear to adequately describe the relationships between these oxygenated species and fuel composition. Data are given for a platinum oxidation catalyst-equipped vehicle, operated under nonoptimal conditions. The results of experiments designed to show the effects of restricted secondary air, high temperature excursions, and short-term exposure to fully leaded fuel on hydrocarbon and oxygenate mass and composition are discussed.