The rate constants and products for the reactions of Cl-(D2O)(n) + CH3Br (n = 1-3) have been measured. The n = 1 reaction was studied from 238 to 478 K. The rate constant is well described by k = (6.0 x 10-10) exp(-1270/T) cm3 s-1. We determined the reaction mechanism to be ligand switching to produce Cl-(CH3Br) followed by thermal decomposition of the complex. Cl-(CH3Br) decomposition produces greater than 90% Br- + CH3Cl with the remainder being Cl- + CH3Br. The Cl-(CH3Br) + He rate constant is well described by k = (4.5 x 10-10) exp(-2260/T) cm3 s-1. RRKM theory was used to model the decomposition of Cl-(CH3Br). The results are consistent with our experimental results if a central barrier height of 22.5 kJ mol-1 is used. The n = 2 and 3 reactions also proceed by ligand switching followed by thermal decomposition. The n = 2 reaction was studied from 203 to 298 K. The rate constant is well-described by (4.4 x 10-9) exp(-1329/T) cm3 s-1. The main product observed was Cl-(D2O) with a smaller amount of Cl-(D2O)(CH3Br) also detected. The n = 3 reaction was studied from 188 to 203 K. The reaction was about a factor of 1.8 faster than the n = 2 reaction. The main product was Cl-(D2O)2 with smaller amounts of Cl-(D2O)2(CH3Br) and Cl-(D2O)(CH3Br) also observed.