An optical method (microscopic holographic interferometry) has been used to visualize concentration changes in the vicinity of the membrane surface while a polyethylene glycol 2000 (PEG) solution was ultrafiltered with a cellulose acetate membrane. This interferometric technique is based on the fact that changes of refractive index, which are associated to changes of concentration, can be visualized as interference fringes. Interferograms obtained in different experimental conditions (initial PEG 2000 concentration range from I to 10 kg/m(3) and transmembrane pressure range from I x 10(5) to 3 x 10(5) Pa) as well as permeate flux are presented. Concentration profiles in the polarized layer near the membrane surface have been determined from these interferograms. In all cases, after about 20 min of ultrafiltration process, the profiles remained stable and a "pseudo-steady state" seemed to be reached. Moreover, solute concentration in the vicinity of the membrane did not reach much higher values than the initial concentration. (C) 2003 Elsevier B.V. All rights reserved.