The series of (one-electron) reductions of methylviologen (MV(2+)) intercalated into zeolite-Y by various carbonylmanganate donors [C+Mn(CO)(4)L(-), L = CO, P(OPh)(3)] are very selective and highly dependent on the size/ shape of the counterion C+, although the same electron transfers carried out (homogeneously) in solution always occur spontaneously, irregardless of C+. For example, the complete reduction of MV(2+) extensively doped into zeolite-Y proceeds rapidly and quantitatively when the Na+ salts of the carbonylmanganates are employed as the reductants, but only to a very limited extent (1%) when the large PPN+ [bis(triphenylphosphine)iminium] salts of the carbonylmanganates are employed. The medium-size tetraethylammonium (TEA(+)) salt of Mn(CO)(4)P(OPh)(3)(-) slowly effects an intermediate conversion (80%). Based on the fact that the large phosphite-substituted Mn(CO)(4)P(OPh)(3)(-) donor cannot enter the supercage of zeolite-Y, we propose interfacial electron transfer from the carbonylmanganate to the MV(2+) acceptor to occur only at the zeolite periphery. Importantly, the strong dependence of the further progress of the redox reaction with decreasing size of the cation C+ (i.e., shape selectivity) predicts that electron conduction throughout the zeolite framework requires the simultaneous transport of these cations in order to effect the complete reduction of all the encapsulated MV(2+), as presented in Chart 5.