The supercage of zeolite-Y and the restricted channels of zeolite-L, mordenite, and mazzite act as shape-selective hosts to a series of brightly colored charge-transfer complexes [A(+), D] assembled in situ by the intercalation of various aromatic donors (D) into the ion-exchanged (colorless) zeolites with different pyridinium accepters (A(+)). Upon the deliberate introduction of water (vapor) into the variously colored zeolites, the diffuse reflectance spectra suffer pronounced spectral shifts of the charge-transfer bands, the magnitude of Delta h nu(CT) being uniquely dependent on the molecular size/shape of A(+) and D, as well as the dimensions of the zeolite cavities. In accordance with the bond compression model of intermolecular charge-transfer complexes, the bathochromic shifts to lower energies (+Delta h nu((CT)) are ascribed to hyperbaric effects induced by the absorption of water within the constricted channels of zeolite-L, mordenite, and mazzite (but not in the larger supercage of zeolite-Y). The importance of water coordination is also underscored by the unusual hypsochromic effects (-Delta h nu(CT)) that are observed when cyano-substituted pyridinium accepters o-CP+ and p-CP+ are doped into zeolite-Y (but not into zeolite-L, mordenite, or mazzite). These results thus show that the intermolecular charge-transfer complexes [A(+), D] are shape-selective and can be used as quantitative visual probes for water absorption into zeolite cavities, and they provide the experimental basis for further theoretical analysis.