We show that the radical cations of adamantane (C(10)H(16)(*+), 1H(*+)) and perdeuteroadamantane (C(10)D(16)(*+), 1D(*+)) are stable species in the gas phase. The radical cation of adamantylideneadamantane (C(20)H(28)(*+), 2H(*+)) is also stable (as in solution). By using the natural (13)C abundances of the ions, we determine the rate constants for the reversible isergonic single-electron transfer (SET) processes involving the dyads 1H(*+)/1H, 1D(*+)/1D and 2H(*+)/2H. Rate constants for the reaction 1H(*+)+1D <==> 1H+1D(*+) are also determined and Marcus' cross-term equation is shown to hold in this case. The rate constants for the isergonic processes are extremely high, practically collision-controlled. Ab initio computations of the electronic coupling (H(DA)) and the reorganization energy (lambda) allow rationalization of the mechanism of the process and give insights into the possible role of intermediate complexes in the reaction mechanism.