To study the formation of atmospheric sulfate, nitrate and chloride particulates in Beijing, China, 14 sets of measurements of the concentrations of SO2, HNO3, NH3 and size-segregated particles were made in the summer of 2001 and the spring of 2002. Because of the very different humidity levels in Beijing in the summer and the spring, significant differences in the size characteristics and in the formation mechanisms of these ions were found. In the summer, the major fraction of sulfate was present in the fine mode with a mass median aerodynamic diameter (MMAD) of 0.7±0.1μm. Its formation was attributed to in-cloud processing, which was supported by the observation of a large mole ratio of the sulfate at 0.54-1.0μm to SO2, with a maximum value of 3.0. The major fraction of nitrate sometimes appeared in the fine mode with an MMAD of 0.7±0.1μm and sometimes in the coarse mode with an MMAD of 6.0±1.5μm. The coarse mode nitrate was associated with Ca2+. The major peak of chloride, in most cases, appeared in the coarse mode. Chloride was expected to have formation mechanisms similar to nitrate formation in the coarse mode. In the spring, the major fraction of sulfate was present in the fine mode with an MMAD of 0.45±0.05μm and its formation was ascribed to non-cloud heterogeneous processes. A small mole ratio of the fine mode sulfate to SO2, with a maximum value of only 0.06, was observed. Nitrate and chloride generally have a similar MMAD as sulfate in the fine mode, suggesting that both may originate from atmospheric processes similar to sulfate formation. Alternatively, they could be formed by gas condensation onto existing sulfate particles. Overall, much more efficient sulfate formation by cloud processing led to a higher sulfate concentration in the summer than in the spring, although the SO2 concentration followed the reverse trend in Beijing. © 2003 Elsevier Science Ltd. All rights reserved.