A series of zeolite Y samples having different contents of entrapped RU(bpy)(3)(2+) (Z-Ru(bpy)(3)) has been prepared. In contrast to the case in homogeneous media, in Y-zeolite the entrapped molecules cannot diffuse and the range of possible intermolecular distances between the complexes is restricted to discrete values. Because of the specific arrangement of interacting species, simplified models can be used in the study of the intermolecular interactions. The results of the present study indicate that intermolecular interactions between the entrapped complexes have a profound effect on the intensity of the electronic emission and kinetics of the excited-state depopulation. Analysis of excited-state lifetime results show that, although isolated Z-*Ru(bpy)(3) molecules decay to the ground state with rates comparable to that observed in aqueous solution, the decay of the excited molecules, which have another Ru(bpy)(3)(2+) or *Ru(bpy)(3)(2+) complex entrapped in an adjacent zeolite supercage, is much more rapid. In the case of excited-state-ground-state interaction, this effect has been attributed to the nonradiative decay enhancement, and in the case of the excited-state-excited-state interaction, it has been attributed to triplet-triplet annihilation via electron transfer.