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Total cross sections for reactive scattering and collisional electron detachment have been measured for collisions of F− and Cl− with H2, D2, and HD. The collision energy extends from below the energetic thresholds for the processes studied up to a laboratory energy of about 300 eV. Reactive scattering is found to be the dominant inelastic channel for the case of the F− projectile. Isotope effects are observed in all cross sections. Electron detachment of F− is found to occur by two distinct mechanisms. A striking difference in the reactive and detachment cross sections is observed when Cl− is substituted for F− in that the electron detachment cross section is generally larger than that for reactive scattering. As in the F− case, isotope effects are also present for the Cl− projectile. The isotope effects observed for electron detachment of Cl− suggest that detachment cannot be described by any unique mechanism.