The rates of anionically promoted oxy-Cope rearrangement of five structurally related exo-norbornanols have been determined. A critical difference in the substitution pattern of these substrates is the presence or absence of oxygen or sulfur functionality at the terminal carbon of the bridgehead vinyl group and the geometric nature of the olefinic center. The results convincingly establish that although an OPMP substituent exerts, at best, a very modest decelerative kinetic effect, the presence of the PhS substituent causes rearrangement to occur approximately 10(3) times faster by lowering the activation energy to the extent of 3-4 kcal/mol. This remarkable divergence in behavior is shown to correlate well with the highly dissociative nature of the transition state that is predicted computationally.