Decisions on controlling emissions to reduce atmospheric aerosol concentrations require accurate information on ambient concentrations, composition and sources. This paper shows how filter sampling of ambient aerosols, followed by multi-element analysis of the filters and factor analysis of the data, were used to identify sources in the St. Louis area. Nearly 400 filters, collected at 12 sites in the St. Louis area from 1973–75 as part of project METROMEX, were analyzed by ion-excited x-ray fluorescence for 10–20 elements. Factor analyses were performed separately for each site, revealing a number of common area-wide sources and several local sources. Widespread sources include soil dust and flyash, secondary sulfates, auto exhaust and metals. Local sources identified include a titanium pigment plant and a secondary lead smelter. Abstract Decisions on controlling emissions to reduce atmospheric aerosol concentrations require accurate information on ambient concentrations, composition and sources. This paper shows how filter sampling of ambient aerosols, followed by multi-element analysis of the filters and factor analysis of the data, were used to identify sources in the St. Louis area. Nearly 400 filters, collected at 12 sites in the St. Louis area from 1973–75 as part of project METROMEX, were analyzed by ion-excited x-ray fluorescence for 10–20 elements. Factor analyses were performed separately for each site, revealing a number of common area-wide sources and several local sources. Widespread sources include soil dust and flyash, secondary sulfates, auto exhaust and metals. Local sources identified include a titanium pigment plant and a secondary lead smelter.