The isotope-exchange reactions of D-2 and CD4 With a zeolite model are analyzed using ah initio calculations. It is concluded that these exchange reactions could occur with activation energies in the 25-40 kcal/mol region. The mechanisms for these exchanges are different than those encountered in previous modeling studies of the addition of ethylene, acetylene, and formaldehyde to the acidic sites of zeolites. The addition mechanisms come close to conceptually involving ZO(-),HR(+)-like transition states. However, the exchange-reaction transition states have anionic character for the transmitted moieties, H- and CH3-, respectively, and two protons are strongly attached to the oxygen of the zeolite model. In the case of isotope exchange of water, protonated water and D-2 and CD4, the transition states come closer to involving fully proton-transfer structures.