A systematic study on the effects of ionic environments on the adsorption and diffusion characteristics of bovine serum albumin (BSA) inside/on 100 kDa polyether sulfone (PES) membranes was carried out. Static adsorption of BSA was investigated at the solution pH values of 3.78, 4.78, and 6.80; for the ionic-strengths of 0.01 and 0.1 M. Isotherms for BSA adsorption on PES membranes were correlated by the Freundlich equation. The maximum adsorption occurred at the pH value below the isoelectric point of the protein. With increasing ionic strength, the adsorbed protein on the membrane decreased. van der Waals and electrostatic interactions energies between the membrane and the protein were calculated by using the DVLO theory. Diffusion coefficients of BSA inside PES membranes were calculated by employing experimental data evaluated in a diffusion cell along with the data of adsorption isotherms into a developed model based on mass-balance. To detect the structural changes occurred, membrane surfaces were analysed by FT-IR-ATR measurements and SEM images. The results of the adsorption and diffusion experiments showed that adsorption and diffusive characteristics of BSA in 100 kDa PES membranes are agreed well with the theoretical calculations. (c) 2005 Elsevier B.V. All rights reserved.