Target transformation factor analysis (TTFA) has been applied to an analysis of a subset of the aerosol-composition data acquire during the Regional Air Pollution Study (RAPS) for St. Louis, Missouri. The RAPS program collected a large number of samples with ten, continuously operated, dichotomous samplers from March 1975 to March 1977. The purpose of the present study was to evaluate the capability of TTFA to resolve sources of airborne particulate matter in a set of ambientaerosol samples. In order to give each element a more equal weight in the identification of sources, a weighting scheme has been added to the target transformation rotation procedure. The weighted rotation produces a more sensitive source identification and has enhanced the resolution of sources with similar element profiles. To determine the most appropriate way to apply TTFA, two separate sets of the data were analyzed: (1) All the samples collected during July and August 1976 at a single station, and (2) All the samples collected at the ten RAPS stations during a single week. Each set of samples were further subdivided into fine- and coarse-faction subsets and analyzed separately. Because of the large number of missing values below detection limits, it was necessary to exclude a sizable fraction of the data from the analyses. Superior results were obtained from the examination of the variation in aerosol composition with time at a single location rather than the spatial variation over multiple sites during a shorter time period.