Tripler excited states of pyrene, phenanthrene, and naphthalene have been prepared in zeolite KY by pulsed laser excitation and the influence of coadsorbed water on quenching of these triplet stales by ferrocene and the immobile ferrocene derivative ferrocenylmethytrimethylammonium cation (FcMN) has been studied by transient absorption spectroscopy. To make these measurements, probe loadings in KY have been kept deliberately low such that quenching involves migration of molecules from one supercage to another. In all cases, maxima in bimolecular triplet quenching rates versus the number of H2O per supercage (N-sc) were observed at N-sc approximate to 4, a value which coincides with the number of H2O molecules that fill the sodalite cages, For pyrene quenching by FcMN, the effect of 4 H2O/sc was particularly dramatic, where the quenching rate increased by 3 orders of magnitude relative to dehydrated KY. Above N-sc = 4, further additions of H2O lowered the rate of quenching. The rate of triplet quenching by FcMN, where quenching results from motion of the aromatic probes in their triplet state to FcMN, generally followed the trend Np-3* > Ph-3* much greater than Py-3*, This trend, as well as the influence of coadsorbed H2O on quenching, have been explained by the effects of adsorption and molecular size on diffusion of these molecules in KY.