A year-long field study to characterize the ionic species in PM2.5 was carried out in Shanghai and Beijing, China, in 1999-2000. Weekly samples of PM2.5 were collected using a special low flow rate (0.4lmin - 1 ) sampler. In Shanghai, SO 4 2 - NO 3 - and NH 4 + were the dominant ionic species, which accounted for 46%, 18% and 17% of the total mass of ions, respectively. Local SO 2 emissions were an important source of SO 4 2 - in PM2.5 because the SO 4 2 - concentration was correlated with the SO 2 concentration (r=0.66). The relatively stable SO 4 2 - /SO 2 mass ratio over a large range of temperatures suggests that gas-phase oxidation of SO 2 played a minor role in the formation of SO 4 2 - . The sum of SO 4 2 - and NO 3 - was highly correlated with NH 4 + (r=0.96), but insufficient ammonium was present to totally neutralize the aerosol. In Beijing, SO 4 2 - , NO 3 - and NH 4 + were also the dominant ionic species, constituting 44%, 25% and 16% of the total mass of water-soluble ions, respectively. Local SO 2 emissions were an important source of SO 4 2 - in the winter since SO 4 2 - was correlated with SO 2 (r=0.83). The low-mass SO 4 2 - /SO 2 ratio (0.27) during winter, which had low humidity, suggests that gas-phase oxidation of SO 2 was a major route of sulfate formation. In the summer, however, much higher mass ratios of SO 4 2 - /SO 2 (5.6) were observed and were ascribed to in-cloud sulfate formation. The annual average ratio of NO 3 - /SO 4 2 - was 0.4 and 0.6 in Shanghai and in Beijing, respectively, suggesting that stationary emissions were still a dominant source in these two cities.