The extension of spectrofluorimetric hydrodynamic voltammetry for the study of heterogeneous reactions occurring at solid/liquid interfaces is described. In particular the rate of migration of the cationic species H+ and K+ within a thin organic film containing the fluorescent probe N-(5-fluoresceinthiocarbamoyl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (F-362) as the triethylammonium salt dispersed in a large excess of 1-trimethylammonium 2,3-di [C14-C18 acyloxy] propane chloride (HEQCl) may be followed through the quenching of the F-362 fluorescence by the cation. Two different types of experiment are described. In the first the organic film is supported on the silica wall of a channel flow cell, and the migration of H+ cations into the film, induced through their stepwise formation at an electrode located upstream of the organic layer. In the second the film is coated onto the channel electrode surface, and migration of K+ ions induced by changes in the electrode potential. In each case the diffusion coefficient of the cation within the membrane may be inferred from the fluorescence transient.