To understand the characteristics and sources of water-soluble organic aerosols (OA) in North China, we studied diacids, oxoacids and α-dicarbonyls in summer- and winter-time fine (PM) aerosols collected from Tianjin. Oxalic (C) acid found to be the most abundant diacid species, followed by succinic (C), malonic (C) and sebacic (C) acids, respectively. Glyoxylic (ωC) 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 PM were exactly the opposite. On average, total diacids carbon accounted for 2.93% in total carbon and 3.31% in organic carbon (OC) in summer and 1.79% and 1.98%, respectively, in winter. Their contribution to water-soluble OC was almost same in both seasons (5.46% and 5.31%, respectively). Molecular distributions, mass ratios of selected diacid (C, C, M, F C, Ph and C) species and the linear relations among the selected species (including ΣC-C and ΣC-C) and with inorganic markers (K and SO) implied that the diacids and related compounds are mainly originated from coal combustion and biomass burning emissions and produced in the atmosphere by both photochemical reactions at regional scale in both the seasons and aging during long-range transport, particularly in summer. This study also revealed that diacids and related compounds and WSOC are increased with increasing SO and they produced in aqueous phase, warranting the need of further reduction in SO and NO to control the water-soluble OA loading over North China.