A program to measure and characterize exhaust particulate emissions from a vehicle equipped with prototype catalyst systems was carried out. Nine catalysts (four monolithic oxidation catalysts, three pelleted oxidation catalysts, and two NOx reduction catalysts) were screened on three test fuels. The oxidation catalysts were representative of those used on 1975 production vehicles; the NOx reduction catalysts, candidates for use in future automotive emission control systems. The NOx catalysts were tested without oxidation catalysts in order to determine their contribution to particulate emissions. The fuels variables studied were additive, sulfur, and aromatic levels. Particulate emission rates and size distributions were measured for each catalyst/fuel combination. Analyses for sulfate, carbon, bound water, nine metals, organic nitrogen compounds, and organic sulfur compounds were also carried out.Sulfuric acid aerosol was produced along with increased quantities of metal containing particulates, especially of iron and zinc, when the vehicle was equipped with an oxidation catalyst. The mass median equivalent diameter of oxidation catalyst exhaust particulate decreased relative to the non-catalyst or NOx reduction catalyst cases, because sulfuric acid was the predominant component. The particle size distribution from a NOx reduction catalyst-equipped vehicle was similar to that of an unequipped vehicle.No nitrogen or sulfur-containing organic particulate matter was found.