A double many-body expansion potential energy surface is reported for the electronic ground state of HS2 by fitting accurate multireference configuration interaction energies calculated with aug-cc-pVTdZ and aug-cc-pVQdZ basis sets upon separate extrapolation of the complete-active-space self-consistent field and dynamical correlation components of the total energy to the complete basis set limit. The major topographical features of the potential energy surface are examined in detail, and the model function is used for thermalized calculations of the rate constants for the S + SH -> H + S-2 reaction at 298 and 400 K. A value of (1.44 +/- 0.06) x 10(-11) cm(3) s(-1) is obtained at 298 K, providing perhaps the most reliable estimate of the rate constant known thus far for such a reaction.