The minimum energy reaction paths and secondary kinetic isotope effects (KIE) for the Cope rearrangements of cis- 1,2-divinylcyclobutane and cis- 1,2-divinyleyclopropane obtained by (U)B3LYP calculations are reported. Both reactions proceed through endo-boatlike reaction paths, and have aromatic transition states. The predicted activation energies are in agreement with the experimental data. The reaction paths of the rearrangements are intervened by enantiomerization saddle points of the products (and the reactant in the case of divinylcyclobutane). The calculated KIES are similar in the two systems, and consistent with the geometries of the transition structures. There is computational evidence that the isotope effect associated with the conversion of a pure sp(2) C-H bond into a pure sp(3) one might be the same in all molecules. The predicted KIES agree with experiment for divinylcyclopropane, but not for divinylcyclobutane.