The excited state intramolecular proton transfer and physical proper-ties of 7-hydroxyquinoline are studied in various solutions and heterogeneous systems by measuring steady state and time-resolved fluorescence, reflection and NMR spectra. Proton transfer is observed only in protic solvents owing to its requirement of hydrogen-bonded solvent bridge for proton relay transfer. The activation energies of the proton transfer are 2.3 and 5.4 kj/mol in CH3OH and in CH3OD, respectively. Dimers of normal molecules are stable in microcrystalline powder form and undergo an extremely fast concerted double proton transfer upon absorption of a photon. consequently forming dimers of tautomer molecules. In the supercage of zeolite NaY, its tautomeric form is stable in the ground state and does not show any proton transfer.