The ground and first and second (1)A" potential surfaces of methyl mercaptan (CH3SH) are calculated as a function of the C-S and S-H bond coordinates using the ab initio effective valence shell Hamiltonian (H(n)u) method. The computations for this highly nontrivial system provide the first serious tests for choosing restricted valence spaces and for computing global potential energy surfaces with the H(n)u methods. The quasidegeneracy constraints on the H(n)u method suggest choosing a valence space which consists of the two (3a" and 10a’) highest energy occupied orbitals in the ground state and the three lowest a’ unoccupied orbitals. The global potential surfaces are computed with a modest basis, but larger basis set computations at selected geometries test convergence for vertical excitation energies, ionization potentials, and C-S and S-H bond energies. The calculations are compared to both experiment and other calculations for this system. The computations assist in the interpretation of CH3SH photodissociation dynamics observed by Butler and co-workers.