Fifty-eight samples were collected in Qingdao from November to December 2012 using a PM2.5 sampler. Concentrations of acidic gases, ammonia and their partner ions in PM2.5 were determined, and the correlation between concentrations and the gas-particle partitioning were analyzed. The averaged concentrations of HNO3, HNO2, HCl, SO2 and NH, in atmosphere samples of Qingdao were 1. 36, 1. 64, 1. 46, 27. 58 and 2. 95 µg.m-3, respectively, for the concentrations of NO3-, NO2, Cl-, SO(4)2- and NH: in PM2, were 6. 49, 0. 12, 1. 95, 9. 36 and 6. 04 µg.m-3, respectively. When the measurements made in haze days, compared to those made in clear days, the concentration of these ions in PM2.5 increased more than those of acidic gases and ammonia; In foggy days, the concentrations of gases were lower than those in clear days except HNO2, while the reverse was true for the concentrations of all particulate species in PM2.5. During whole study period, the NH3 level was high and the NH4+ in PM2.s in Qingdao was mainly present as (NH4)2SO4. The analysis results suggested that formation of N03 were mainly determined by HNO3. To further explore the control factors of these species in PM2.5, the thermodynamic equilibrium model of ISORROPIA I was employed. Our sensitivity tests showed that the formation of NH4+ was less sensitive to the change of total NH3 (TNH3), but sensitive to the changes of total H2 SO4(TSO4) and total HNO3 (TNO3). The formation of NO3- and SO(4)2- was sensitive to the changes of TNO3 and TSO4, respectively. Taken together, our findings implied that the reduction of TNO3 and TSO4 rather than TNH3 appears to be more effective in lowering mass concentrations of PM2.5 in Qingdao.