Particles with aerodynamic diameters <10 microm (PM(10)) and particles with aerodynamic diameters <2.5 microm (PM(2.5)) were sampled during summer 2006 in Beijing and mass concentrations, water-soluble ionic compounds concentrations, and acidic buffer capacity were analyzed. Results show that the mass concentration ranges of PM(10) and PM(2.5) were from 56.4 to 226.6 microg/m(3) and from 31.3 to 200.7 microg/m(3) during sampling days, respectively. Concentrations of F(-), Cl( -), NO3(-), NO2(-), SO4(2-), Ac(-), Ca(2+), Na(+), K(+), Mg(2+), and NH4(+) in particles were analyzed by ion chromatography. Microtitration was adapted to determine the acidic-basic property and the change of the buffering systems in different pH of the aqueous solution in which the PM is suspended. The major alkalinity and buffer capacity of particles were analyzed and calculated. The average carbonate buffer capacity was 0.3 mmol/g in PM(2.5) and 0.7 mmol/g in PM(10). The average acetic acid buffer capacity was 0.1 mmol/g in PM(2.5) and 0.3 mmol/g in PM(10). Carbonate and acetic acid are the main species for the buffer capacity in the particle phase. The average mass of carbonate was 71.0 mg/g in PM(10) and 46.7 mg/g in PM(2.5). The average mass of acetic acid was 11.2 mg/g in PM(2.5) and 20.0 mg/g in PM(10).