Electron-phonon coupling in the monocations of cubic clusters such as cubane (CH)(8) and octasilacubane (SiH)(8) are studied. The total electron-phonon coupling constants for the monocations (l(HOMO)) are estimated and compared with those for the monoanions (l(LUMO)) in cubane and octasilacubane. The calculated l(HOMO) value for cubane (0.766 eV) is much larger than that for octasilacubane (0.327 eV). And the l(HOMO) values are larger than the l(LUMO) values both in cubane and octasilacubane. The frequencies (omega(ln,HOMO)) of the vibrational modes which play an essential role in the electron-phonon interactions and the possible superconducting transition temperatures (T(c)s) are estimated on the basis of the hypothesis that vibronic interactions between the highest occupied molecular orbitals (HOMO) and intramolecular vibrations would play an essential role in the occurrence of superconductivity in positively charged nanosized molecular systems. The phase pattern difference between the delocalized t(2g) HOMO in cubane and the t(1u) HOMO localized on silicon atoms in octasilacubane as well as the molecular weights difference between cubane and octasilacubane is the main reason why the l(HOMO) and omega(ln,HOMO) values for cubane are much larger than those for octasilacubane, and the reason why the possible T(c)s for the monocation of cubane are estimated to be much larger than those for the monocation of octasilacubane. In a similar way, the T(c)s for the monocation are estimated to be larger than those for the monoanion in cubane. The important vibronic interaction effects originating from the orbital interactions between carbon and hydrogen atoms due to the delocalized electron density in the t(2g) HOMO in cubane are the main reason why the T(c)s for the monocation are estimated to be larger than those for the monoanion in cubane. (C) 2003 American Institute of Physics.