In this Letter, MP2, DFT and CASSCF calculations were carried out to investigate proton transfer reactions of 8-hydroxyquinoline (8-HQ) and its one-water complex (8-HQ-H2O). Since the forward reactions from the normal form (N) to the tautomer form (T) in the ground state have a considerable barrier and the reverse processes nearly have no barrier, the tautomerization reactions from N to T proceed with little probability in the ground state of 8-HQ and 8-HQ-H2O. After photoexcitation, the excited-state proton transfer reactions proceed very easily for both 8-HQ and 8-HQ-H2O, which are mainly responsible for lack of fluorescence for 8-HQ in aqueous solution. (C) 2002 Elsevier Science B.V. All rights reserved.