To understand the characteristics and sources of organic aerosols (OA) in North China, we studied diacids, oxoacids and α-dicarbonyls in summer- and winter-time fine aerosols (PM2.5) collected from Tianjin. Oxalic (C2) acid was found to be the most abundant diacid species, followed by succinic (C4), malonic (C3) and sebacic (C8) acids, respectively. Glyoxylic (wC2) was the most abundant oxoacids followed by pyruvic acid. Concentrations of total diacids, oxoacids and α-dicarbonyls in winter were 2~3 times higher than those in summer, but their mass fractions in PM2.5 were exactly the opposite. On average, total diacids carbon accounted for 2.9% in total carbon and 3.3% in organic carbon (OC) in summer and 1.8% and 2.0%, respectively, in winter. Their contributions to water-soluble OC (WSOC) was almost the same in both seasons (5.5% and 5.3%, respectively). Molecular distributions, mass ratios of selected diacid (C3, C4, M, F C6, Ph and C9) species and the linear relations among the selected species (including åC2-C4 and åC8-C12) and with inorganic markers (K+ and SO42-) implied that the diacids and related compounds are mainly originated from coal combustion and biomass burning emissions and produced in the atmosphere by both in-situ photochemical reactions at local scale and aging during long-range transport in both summer and winter. This study revealed that diacids and related compounds and WSOC are increased with increasing SO42- and they are produced in aqueous phase, implying the need of reduction in oxidants (NOx and SO2) emissions to control the water-soluble OA loading over North China.