Measurements are reported on the bromination of 2-carbethoxycyclopentanone in the temperature range -20 to +25 degrees C. The medium used throughout was aqueous 5$\cdot $2 M sodium bromide solution, and the base catalysts investigated were, besides the solvent, the fluoride and acetate ions. In all cases the observed activation energies and collision numbers are greater than for similar reactions in solutions of ionic strength 0$\cdot $2. At low temperatures the Arrhenius plots exhibit a curvature which is greatest for catalysis by fluoride ion. This deviation from linearity is shown to be consistent with predictions made by supposing that the motion of the proton across the energy barrier is markedly non-classical. The dimensions of the energy barriers involved are recalculated upon the basis of earlier work on the hydrogen isotope effect in this reaction, by the application of a more precise theoretical treatment than that used previously. It is shown that this interpretation indicates that the fluoride ion-catalyzed reaction should show a very large hydrogen isotope effect in the region of -20 degrees C.