The effects of pH, hypochlorite and chloride ion concentration, temperature, and ionic strength on the kinetics and mechanism of decomposition of concentrated hypochlorite ion and the formation of chlorate ion and oxygen in the pH 9-14 region has been investigated. In the absence of catalytic levels of transition-metal ions, the rate of chlorate ion formation is 8.7 times faster than the rate of oxygen formation for the concentration range of 0.7-3.0 M hypochlorite ion with various levels of chloride ion in 0.001-1.0 M OH(-) at 15-55 degrees C. Under these same conditions, the equation that describes the effect of temperature and ionic strength on the decomposition of OCl(-) is the following: log k(2) = 0.149&mgr; + log[2.083 x 10(10)T exp(-1.018 x 10(5)/RT) exp(-56.5/R)] where k(2) has units of M(-)(1) sec(-)(1) and &mgr; is between 1 and 6 molar. Numerical simulation of the decomposition of OCl(-) was carried out while taking into account the effect of each of the above experimental variables. Evidence is presented suggesting chloride ion catalyzed decomposition of hypochlorite ion in the pH 9-10 region that is in addition to the contribution of chloride ion to the ionic strength.