Proton transfer process of pyranine (8-hydroxypyrene-1,3,6-trisulfonate, HPTS) at the surface of a cationic micelle (cetyl trimethyl ammonium bromide, CTAB) has been studied by emission spectroscopy. At the CTAB micellar surface, pH is similar to9.5 and the ground state pK(a) of HPTS is 10.2. These are higher compared to bulk water (pK(a) = 7.4 and pH 7). At the surface of the micelle, emission of HPTS is quenched by acetate ion with a bimolecular quenching constant, 3.5 x 10(7) M-1 s(-1). This is markedly slower than the excited state proton transfer in a solvent separated HPTS-water-acetate complex in bulk water. (C) 2004 Elsevier B.V. All rights reserved.