Photophysical studies of naphthalene confirm that aromatic molecules tend to aggregate within cation exchanged Y zeolites. Ground-state aggregation is traced to the presence of cation-aromatic pi-interaction. Solvents that can coordinate to the cation "turn off＇ the cation-aromatic interaction, and consequently aggregation does not occur in zeolites that are impregnated with the above solvents. The solvent that exhibits a maximum in such an effect is water. MP2 calculations on cation-benzene dimer indicate that cation-pi-interaction results in stabilization of the pi-stacked benzene dimer. Results of MP2 calculations are consistent with the formation of ground-state pi-stacked aggregates of naphthalene molecules within Y zeolites.