The spatial and temporal variability of SO4(-2) concentrations in precipitation over the eastern United States during the period 1981-1986 was examined through the use of principal component analysis. Application of Kaiser's Varimax orthogonal rotation led to the delineation of seven contiguous subregions, each displaying statistically unique SO4(-2) concentration characteristics. These seven statistically significant modes of variability, which together accounted for 74.2% of the total variance, corresponded well with major SOx emission patterns. Examination of the time series associated with subregion revealed a general seasonality in which periods of high concentrations are more likely during the summer, while periods of low concentrations are more likely during the winter. This seasonal cycle, however, was more prevalent in those subregions which contained few major emissions, and was less prevalent and often obscured by perturbations in those subregions which contained major emissions.