The Rio de Janeiro Aerosol Characterization Study was a joint U.S.-Brazilian project undertaken 1) to provide more complete information on the chemical composition of the wintertime Rio aerosol, with emphasis on its carbonaceous and sulfate components and 2) to compare the chemical composition of the Rio aerosol with those of urban U.S. aerosols.Concentration of inhalable (D50 = 15 μm) particulate matter (IPM) in Rio de Janeiro averaged 80.7 and 58.6 μg/m for day and night samples, respectively, at a site impacted by heavy motor vehicle traffic, during a wintertime sampling period. The average IPM concentration was higher than concentrations of IPM in many U.S. cities. Atmospheric concentrations of extractable organic matter (EOM) and polycyclic aromatic hydrocarbons were similar to those measured in eastern U.S. cities during the wintertime while organic and elemental carbon concentrations were somewhat higher than those reported for most urban aerosols in the United States. Concentrations of sulfate were only about one-third of those measured in the United States.The overall composition of the Rio aerosol also differed from those of U.S. urban aerosols. Sulfate (as NH4HSO4) was only 7.7 percent of IPM, compared to 20–40 percent for most U.S. cities. EOM was only 24 percent of the Rio IPM as compared to 27–36 percent of wintertime IPM samples from urban Mew Jersey sites. Nonpolar organics constituted a greater proportion of total Rio EOM, as compared to U.S. cities, and the distribution of three EOM fractions was similar to that observed for samples collected in a traffic tunnel indicating a strong impact from motor vehicles. The ratio of organic to elemental carbon (OC/EC) in Rio was about 1 and was lower than the ratios observed in most U.S. cities. This appears to be a consequence of the 8–10 percent diesel powered buses in the Rio traffic mix.