Abstract: The acidity of precipitation in equilibrium with atmospheric CO2 and SO2 has been calculated for both clean and polluted areas. It shows that in temperate latitudes the pH value of precipitation in clean areas is below 5 due to the existence of SO2 and COa. In Beijing, where the aerosol concentration is high, the pH value of precipitation is increased by the reactivity of chloride and Ca0 with acidic substances. The pH value of precipitation in Beijing is closely relaled to the liquid water content in cloud, the rainfall amount and the concentralions of particulate Cl and Ca in the atmosphere. This mechanism may help to explain the non-acid rain in Beijing where the concentration of S02 is high.