A method employing size-exclusion chromatography (SEC) with Total Organic Carbon detection is developed to isolate and quantify the water-soluble organic carbon (WSOC) component of ambient aerosol particles by acid, neutral, and basic functional groups. The method provides unique quantitative insights into the characteristics and possible sources of a large fraction of the organic aerosol. The SEC is combined with a XAD-8 method that separates WSOC into hydrophilic and hydrophobic fractions. Calibrations show the hydrophilic fraction separates into short-chain aliphatic acids (WSOCxp_a), neutrals (WSOCxp_n, e.g., saccharides, polyols, and short-chain carbonyls), and organic bases. Recovered hydrophobic fractions are separated into acids (WSOCxrr_a, e.g., aromatic) and neutrals (WSOCxrr_n, e.g., phenols). 13Carbon--nuclear magnetic resonance on ambient samples support the calibration-based conclusions; however, calibrations only provide a guide to the type of ambient organic compounds expected in each group. Comparisons are made between XAD-8/SEC results from urban Atlanta summer and winter and biomass burning samples. The largest isolated fraction of Atlanta summer WSOC is WSOCxp_a (29% ¿g C/¿g C), suggesting aliphatic acids of less than C4 or C5 are the dominant secondary organic aerosol product. Combined with WSOCxrr_a, these acid groups are a higher fraction of summer organic carbon (20%) than winter (14%). They are correlated with each other (R2 = 0.74), with WSOCxp_n (R2 = 0.61 and 0.52), and with gaseous 24-hour averaged volatile organic compounds linked to mobile sources. In biomass burning samples, neutrals (WSOCxp_n and WSOCxrr_n) dominate the WSOC. Atlanta winter samples have characteristics that appear to be a combination of summer and biomass samples.