Weekly PM2.5 samples were simultaneously collected at a residential (Tsinghua University) and a downtown (Chegongzhuang) site in Beijing from July 1999 through September 2000. The ambient mass concentration and chemical composition of the PM2.5 were determined. Analyses included elemental composition, water-soluble ions, and organic and elemental carbon. Weekly PM2.5 mass concentrations ranged from 37 to 357 μg/m3, with little difference found between the two sites. Seasonal variation of PM2.5 concentrations was significant, with the highest concentration in the winter and the lowest in the summer. Spring dust storms had a strong impact on the PM2.5. Overall, organic carbon was the most abundant species, constituting no less than 30% of the total PM2.5 mass at both sites. Concentrations of organic and elemental carbon were 35% and 16% higher at Tsinghua University than at Chegongzhuang. Ammonium, nitrate and sulfate were comparable at the sites, accounting for 25–30% of the PM2.5 mass.