This work was undertaken by the Bureau of Mines in cooperation with the Environmental Protection Agency to determine photochemical specific reactivities of emissions from automobiles using leaded- premium- and regular-grade fuels and prototype lead-free fuels of comparable octane quality. Emissions were photoirradiated in a 100- ft3 environmental (smog) chamber constructed of aluminum and pyrex glass at three sets of initial hydrocarbon (hc) and oxides of nitrogen (nox) concentrations. Reactivity was measured and expressed in terms of rate-of-NO2 formation (rNO2) and dosages of oxidant, peroxyacetyl nitrate (pan), peroxypropionyl nitrate (ppn), and formaldehyde (hcho). The results indicate that the evaporative emission samples and the evaporative plus exhaust emissions samples from the lead-free fuels have, in general, higher reactivities per gram than those from the leaded fuels. For the samples containing only exhaust emissions, the only trend observed was the lower specific reactivity exhibited by the leaded premium fuel exhaust. These differences in specific reactivity are taken to reflect the differences in hydrocarbon composition of the fuels and their evaporative and exhaust emissions rather than any difference in lead content per se. Work done under an agreement with the Environmental Protection Agency.