Since 1980, the Acid Precipitation in Ontario Study (APIOS) Event Wet/Dry Deposition Network has collected a substantial precipitation chemical data base. Data from the three stations in Dorset, Kingston and London were selected for investigation of the sources of the constituents found in the collected precipitation. Factor analysis was applied to the chemical data sets. Three factors reproduce the concentrations observed in precipitation. These factors represent the acid gas sources (SO 2 and NO x sources), Ca and Mg sources, and Na and Cl sources (marine aerosol). The acid gas sources are responsible for the acidity of the precipitation. To locate these sources in terms of geographic region; air parcel back trajectories corresponding to the precipitation sample time periods are incorporated into factor analysis with the chemical data. This information is in the form of the geographic distribution of back trajectory endpoints. The region in which the trajectory endpoints fall is divided into subregions. New variables are defined as the number of the endpoints in each of the subregions. They reflect the residence time of the air parcels in the subregions. Factor analysis is applied to the combined chemical and endpoint variable data sets. The results show the relationship between the chemical nature of the sources and the geographic regions that the air parcels passed. The potential source contribution function (PSCF) is another approach to locate the sources. PSCF is a conditional probability function. It can be calculated for each 1° longitude by 1° latitude cell by dividing the number of trajectory endpoints that correspond to samples with factor scores or pollutant concentrations greater than specified values by the number of total endpoints in the cell. Source areas are indicated by high PSCF values. Both of the approaches suggest that the Ohio River Valley and some areas along the east coast are major source areas for the observed precipitation acidity in Ontario.