OC through a first-order process characterized kinetically by the activation parameters log AH= 9.8 and EaH= 21.5 kcal/mol. Parallel kinetic work with the 7-deuterio-7- methyloctatriene establishes the Arrhenius parameters log AD= 10.3 and EaD= 23.5 kcal/mol. Thus AD/AH= 3.2 and (EaD-EaH)= 2.0 kcal/mol, and a substantial tunneling component for the [1, 7] sigmatropic hydrogen migration is evident.