Tropospheric aerosols were collected during C-130 aircraft campaign over the east Asia/Pacific region in April to May 2001 and were analyzed for low molecular weight dicarboxylic acids. Homologous series of C2--C5 dicarboxylic acids were detected, for the first time, in the polluted troposphere in the Asian region using aircraft. Oxalic acid (C2) was found to be the most abundant species followed by malonic (C3) or succinic (C4) acid. Total concentrations of C2--C5 diacids (44--870 ng m-3, average 310 ng m-3) are similar to those reported in urban Tokyo near the ground level. Concentrations of oxalic acid showed a positive correlation (r2 = 0.70) with total organic carbon (TOC), although other diacids showed weaker positive correlation. These results suggest that water-soluble dicarboxylic acids and TOC were emitted from similar sources on the ground in the Asian continent and/or produced by the photochemical oxidation of anthropogenic organic compounds in the atmosphere. The diacid-carbons were found to comprise 0.2--3.3% (average 1.8%) of TOC. Water-soluble dicarboxylic acids may play an important role in controlling the chemical and physical properties of organic aerosols in the polluted troposphere over the east Asia and the western Pacific.