Using free-radical hydrogen abstractions of the type X-H + Y-. --> X-. + H-Y as a model for reactions involving simultaneous bond breaking and bond making, we find that the enthalpy of activation may increase or decrease with temperature. This depends on changes in bond dissociation energies and their effect on (a) the enthalpy of reaction and (b) the triplet repulsion between the terminal groups X and Y. The variety of observed curvatures in plots of In(k) vs 1/T for eight typical reactions can be described primarily by these two factors, over temperature ranges greater than 1000 K. The reactions treated are OCH2 + H-., CH4 + H-., H-2 + (CH3)-C-., C2H6 + (CH3)-C-., C2H6 + H-., H-2 + (C2H5)-C-., CH4 + Br-., and H-2 + Br-..