Reaction cross sections, product axial velocity distributions, and potential energy surfaces are presented for the hydrogen atom abstraction reactions S-+RH-->R+HS- (R=H, CH3, C2H5) as a function of collision energy. The observed threshold energy, E-0, for S-+H-2-->H+HS- agrees with the reaction endothermicity, Delta(r)H(0). At low collision energies, the H+HS- products exhibit symmetric, low-recoil-velocity scattering, consistent with statistical reaction behavior. The S-+CH4-->CH3+HS- and S-+C2H6-->C2H5+HS reactions, in contrast, show large excess threshold energies when compared to Delta(r)H(0). The excess energies are partly explained by a potential energy barrier separating products from reactants. However, additional dynamical constraints must account for more than half of the excess threshold energy. The observed behavior seems to be general for collisional activation of anion-molecule reactions that proceed through a tight, late transition state. For RH=CH4 and C2H6, the HS- velocity distributions show anisotropic backward scattering at low collision energies indicating small impact parameters and a direct rebound reaction mechanism. At higher collision energies, there is a transition to HS- forward scattering and high velocities consistent with grazing collisions and a stripping mechanism. (C) 2003 American Institute of Physics.