Abstract: PM_ 2.5 samples were collected at Lianda, Yufangyuan and Huankeyuan in Beijing during 2000—2001, and elements, ions, OC/EC, and organic compounds were determined. A chemical mass balance (CMB) receptor model using PAHs and some inorganic compounds as tracers was applied in this study to apportion source contributions to ambient PM_ 2.5 in Beijing. The results showed that the major sources of PM_ 2.5 in Beijing were coal burning, dust, vehicle exhaust, construction dust, biomass burning, secondary sulfate and nitrate, and organic matter. The variation of source contributions was not significant among the sampling sites, but distinct seasonality was observed for the sources of coal combustion, dust, biomass burning, and secondary sulfate and nitrate. A comparison with the results obtained in 1989—1990 indicated that source contributions to PM_ 2.5 were changing during the decade.