Equilibrium and mass transfer data for the extraction of valeric acid from aqueous solutions containing less than 2.5 wt% of acid with a macroporous fresh resin. Amberlite XAD-4, and containing tri-N-butyl phosphate (TBP) at 25degreesC are reported. An equilibrium model, which takes into account the valeric acid physical adsorption by the resin (Freundlich isotherm) and the reactive extraction with the TBP inside the pores, provided a good correlation of experimental data. Valeric acid extraction rates were comparatively faster under the initial conditions of higher TBP concentration in the resin and lower concentration of acid in the aqueous phase. Intraparticle effective diffusivities were determined using a homogeneous mass-transfer model. A heterogeneous diffusion model which considers parallel diffusion of the TBP-valeric acid complex in the organic phase inside the particle pores and surface diffusion of the valeric acid in the polymeric structure of the resin was employed to derive the pore diffusivity and solid surface diffusivity. Both diffusivities were of the same order of magnitude (10(-11) m(2)/s) showing that both diffusion paths in parallel make significant contributions to the intraparticle mass transfer of valeric acid in the extractant-impregnated resins (EIR).