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Daily PM_2.5 and water-soluble inorganic ions (Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺, Cl⁻, NO₃⁻ and SO₄²⁻) were collected in Xi'an (34.23°N, 108.88°E), China from March 2006 to March 2007. PM_2.5 was collected using battery-powered mini-volume samplers. And the ions were determined by ion chromatography from the measured aerosol mass. The annual average mass concentration of PM_2.5 was found to be 194.1±78.6μgm⁻³, which exceeded substantially the international guidelines for health concerns. The seasonal average mass concentration of PM_2.5 was highest in winter (266.8μgm⁻³) and lowest in summer (138.6μgm⁻³). The three highest abundant ions were SO₄²⁻, NO₃⁻, and NH₄⁺, with average concentrations of 35.6±19.5μgm⁻³, 16.4±10.1μgm⁻³, and 11.4±6.8μgm⁻³, which were accounted for 18.7%, 8.0%, and 5.7% of the PM_2.5 mass, respectively. The major ions were in the species of (NH₄)₂SO₄ …