In an earlier work, rates of reaction of iodide ion with cyclopentylmethyl bromide and cyclohexylmethyl bromide in dry acetone were determined. These two compounds reacted at about the same rate, but there were differences in the entropies of activation and in the energies of activation. Cyclopentylmethyl bromide was found to have the lower entropy of activation. This lower value suggests that it is more difficult for the cyclopentylmethyl bromide molecule to assume the transition state. Since the favorable conformation for the bromomethyl group in cyclohexylmethyl bromide is the equatorial one, and since the bromomethyl group of cyclopentylmethyl bromide is similarly oriented, it was decided to study bromides in which the bromomethyl group is rigidly held in the axial or in the equatorial position. Then, by determining rates of reaction of iodide ion with these bromides, the effects of the different conformations would be apparent.