To clarify the characteristics of the water-soluble organic components in atmospheric aerosols, size-separated aerosol samples were simultaneously collected at a roadside site (R) and a suburban background site (S) in Saitama, Japan, during spring and summer 2007 and winter 2008. Chemical compositions, including water-soluble organic carbon (WSOC), organic carbon, inorganic ionic components, and individual water-soluble organic acids (saturated dicarboxylic acids, unsaturated dicarboxylic acids, ketocarboxylic acids, and dicarbonyls), were analyzed in size-separated samples. The seasonal variation of WSOC in Saitama aerosols was larger than the spatial difference between the two sites, with summer WSOC concentrations being 2.5-2.8 times those in the other seasons. Seasonal average concentrations of the detected organic acids in PM7.0 were 542 ng/m3 (R) and 670 ng/m3 (S). Strong correlations were observed between C2-C5 n-dicarboxylic acids and ambient oxidants. The concentration ratios of individual n-dicarboxylic acids (C2-C5) to elemental carbon were significantly higher in suburban samples than in roadside samples, indicating that the contribution of secondary formation to these acids was larger in suburban samples. During the warm seasons, the concentrations of sulfate, ammonium, WSOC, and individual acids in fine particles were very high, whereas nitrate, chloride, sodium, and calcium concentrations were higher in coarse particles. Comparisons between the two sites showed that secondary formation contributed more to the total amount of particulate water-soluble organic acids in Saitama aerosols than direct emissions from anthropogenic and natural sources. However, vehicle exhaust was also an important source of dicarboxylic acids in Saitama aerosols, especially in the near-roadside environment.