Methyl bromide degradation in sea water can be described by a summation of the hydrolysis and chloride ion exchange reactions. Laboratory experiments covered chloride concentrations of 0.1 to 1.0 mol/l, and temperatures from 20 to 60 ¿C. The first-order hydrolysis rate constant is k(H2O)=(5.7¿0.6)¿1011 e-(12455¿240)/T sec-1 and the second-order chloride ion exchange rate constant deduced from the experiment is k(Cl-)=(1.09¿0.21)¿1013 e-(12724¿560)/T l/mol.sec. At a sea water surface temperature of 21.9 ¿C and a chloride concentration of 0.56 mol/l, the calculated degradation half-life of methyl bromide in sea water is 4 days. At 35 ¿C, &tgr;1/2=22 hr.