Hartree-Fock calculations, utilizing the SV4PP split valence contracted Gaussian basis set for Cl and Br and the 6-31G* basis set for CH3, are used to determine potential energy surface properties for the Cl- + CH3Br --> ClCH3 + Br- S(N)2 nucleophilic substitution reaction. This ab initio information is used in concert with experimental data to develop a multidimensional analytic potential energy function for the Cl- + CH3Br reaction system, by modifying the analytic potential energy function developed previously for Cl-2(-) + CH3Clb --> Cl2CH3 + Cl-b(-). Each atom and degree of freedom is treated explicitly in the analytic function. Transition-state theory calculations are performed with the derived analytic potential to determine rate constants for Cl- + CH3Br --> Cl-...CH3Br association, and for dissociation of the Cl-...CH3Br prereaction complex to reactants Cl- + CH3Br, and isomerization of this complex to the postreaction complex ClCH3...Br-. RRKM calculations are also performed to determine rate constants for dissociation and isomerization of the complexes versus energy and angular momentum. The analytic function developed here for the Cl- + CH3Br --> ClCH3 + Br system has been incorporated into the general chemical dynamics computer program VENUS.