The effects of solvation, isotopic substitution, and temperature on the reactions of F-(H2O)(n = 0-5) with CH3Br have been studied. The reaction of n = 0 produces Bi as the exclusive ionic product and has a rate which is fast, approaching the collision rate, with a slight negative temperature dependence of T-0.9+/-0.1. The reaction of n = 1 is a factor of 3 slower with a slightly larger negative temperature dependence, T-1+/-0.05. The main ionic product is Br-, with smaller amounts of Br-(H2O) also formed. Substituting D2O for H2O causes die n = 1 rate constant to increase slightly. The n = 2 reaction has a much slower rate and produces three ionic products : Br-, Br-(H2O), and F-(H2O)(2)(CH3Br). The n = 3 reaction is immeasurably slow. The rate constants for the n = 4 and 5 reactions are greater than those for the n = 3 reaction. The ionic products of these reactions are F-(H2O)(3) and F-(H2O)(4), respectively.