Extraction and back-extraction of valeric acid in a fixed bed packed with Amberlite XAD-4 resin impregnated with tri-n-butyl phosphate were experimentally studied at 25 degrees C. The effects of the feed flow rate, acid concentration in the feed solution and extractant concentration in the impregnated resin on the breakthrough curves, were investigated. The bed saturation capacity was larger under the conditions of higher extractant concentration in the resin phase and higher acid concentration in the feed solution. A dynamic model that considers intraparticle diffusion and external liquid film diffusion as limiting steps in mass transfer rates was successfully applied. The intraparticle effective diffusivities (10(-9) dm(2) s(-1)) were from one to three orders of magnitude lower than the diffusivities in the external liquid film (10(-8) - 10(-6) dm(2) s(-1)). A fast and complete back-extraction of valeric acid from the saturated bed was carried out with sodium hydroxide solutions. The operational life of the impregnated resin was also studied. (c) 2005 Society of Chemical Industry