As part of the development of receptor models that can be employed in air quality management, the application of target transformation factor analysis (TTFA) has been under active investigation. This method has the advantage of requiring limited a priori knowledge of the number or elemental composition of the sources contributing to the observed airborne particulate composition. The approach has been applied to aerosol composition data taken from the Regional Air Pollution Study (RAPS) of St. Louis, MO. The fine fraction at site 108 near Granite City, Illinois for July and August 1976, was examined in this study. In the data, secondary sulfate aerosol particles accounted for 48% of the mass of the average sample. Motor vehicle emissions accounted for another 29%. Flyash/soil, smelter, and an unidentified source accounted for the remaining particulate mass.