Abstract: Water quality parameters including chlorine to ammonia nitrogen application ratio w(Cl)∶w(N),initial chloramine concentration,organic compound level,nitrite concentration,temperature and pH,which affect chloramines decay,are investigated in pure water and the water of a distribution system.Results show that chloramine can autodecompose in pure water.w(Cl)∶w(N) and initial chloramine concentration influence its decay rate.The rate of chloramine decay is higher in bulk water of the distribution system than that in pure water.Chloramine is more stable with w Cl∶w(N)=4∶1 than that of w(Cl)∶w(N)=3∶1.Initial chloramine concentration is an important factor inducing the high rate of chloramines decay with high initial chloramine concentration.The rate of Chloramines decay increases with increasing organic compound and nitrite concentration.The temperature inpacts on the rate of chloramines decay,which means high decay rate in bulk water with increasing temperature.In addition,the reaction of chloramines decay is a type of acid-catalysis,and lower pH elevates the rate of chloramines decay.