Fine aerosol particles collected in Southeast Asia during 1997 Indonesian forest fires were studied for the concentrations of total carbon (TC), water-soluble organic carbon (WSOC) and low molecular weight dicarboxylic acids (C2--C12) as well as carbon isotopic ratios of TC (Δ13CTC). TC and WSOC showed a large increase during the heavy forest fire event. At the same period, dicarboxylic acids, dominated by oxalic (C2) followed by succinic (C4) and malonic (C3) acids, also showed a concentration increase. Furthermore, the Δ13CTC showed a decrease from ca. -25.5 to -27.5? during an intensified forest fire event, suggesting an addition of organic aerosols derived from C3 plants whose Δ13C are lighter. These results indicate that the aerosol particles in Southeast Asia were significantly affected by the combustion processes of vegetations during the 1997 Indonesian forest fires that were extensively induced by El Nin¿ event. ¿ 1999 American Geophysical Union