The diffusion of sodium dithionite (S2O42-) through polyelectrolyte multilayers of poly(diallyl dimethyl ammonium chloride) (PDADMAC)/poly(styrene sodium sulfonate) (PSS) assembled on colloidal particles with the layer-by-layer technique is studied by means of flow cytometry and quenching assay. Fluorescence is provided by a layer of (7-nitrobenz-2-oxa-1,3-diazol-4yl) amino) hexanoate (NBD)-labeled poly(allyl amine hydrochloride) assembled below the PDADMAC/PSS multilayer. NBD is quenched by a redox reaction with S2O42-. NBD quenching is fast at short times but strongly retarded at longer times. Quenching is faster for PDADMAC as the top layer and for increasing concentrations of S2O42-. The quenching kinetics of NBD is described with a model assuming a non-Fickean diffusion of S2O42-, with diffusion coefficients that depend on time with an inverse power law. Diffusion coefficients show little dependence on the number of layers but are highly dependent on the concentration of S2O42-. Increasing the concentration of S2O42- over 10 mol/m(3) results in a decrease of the diffusion coefficient, more evident at longer times. The non-Fickean behavior for S2O42- diffusion is explained on the basis of the trapping of dithionites in the multilayers.