Effect of association with chloride or perchlorate anions on the fluorescence properties of berberine, a cationic isoquinoline alkaloid, has been studied. Interaction with Cl- caused more efficient fluorescence quenching; it significantly accelerated the radiationless deactivation and slowed down the radiative transition. Combined analysis of spectrophotometric, steady-state and time-resolved fluorescence results provided 1.5 x 10(5) M-1 for the equilibrium constant of ion pairing with Cl- in CH2Cl2. Both ion pairing and enrichment of the microenvironment of berberine in ions led to excited state quenching in solvents of medium polarity, but only the latter effect was observed in the presence of perchlorates in butyronitrile. (C) 2007 Elsevier B.V. All rights reserved.