Aerosols are sampled at Institute of Atmospheric Physics, Chinese Academy of Sciences from July to October 2008.The mass concentrations of water-soluble inorganic ions are determined with ion chromatogram(IC).This aims to investigate the composition, characteristics of mass concentration variation, relevance and form of the main water-soluble inorganic ions and their sources.The results show that SO₄^2-, NO₃^- and NO₄⁺ are the main water-soluble inorganic ions in the aerosol samples, and they are mainly in the form of (NH4) 2SO₄ and NH₄NO₃ in aerosol particles.The correlation coefficients between SO₄^2-, NO₃^- and NO₄⁺, Ca²⁺ and Mg²⁺ are high, showing a good homology among these ions.Mass concentration ratio of NO₃^-/SO₄^2- indicates that, before 2008 Beijing Olympics, the stationary source(coal burning) is the main pollution source for water-soluble species compared to the mobile source(vehicle), which is opposite to those during the Olympics and after it.Factor analysis shows that vehicle, coal combustion, soil, building dust and biomass burning are the main sources for Beijing air pollution during the sampling period.