The membrane separation processes are commonly used for concentrating and purifying colloidal dispersions in industry. The main limitation of this process is the accumulation of matter in the neighborhood of the membrane surface. The concentration polarization and/or deposit formation related to this accumulation of matter are the dominant causes of flux decline and control the filtration performance. Consequently there is tremendous interest in understanding mechanisms involved in the structural evolutions of the polarization layer at the colloidal length scales. It would allow devising appropriate steps to reduce the structuring effect. During the past decade few studies [1] have been focused on in-situ monitoring techniques to observe the concentration polarization and fouling phenomena. The focus of this work is in-situ characterization of the induced structures of colloidal dispersions in the vicinity of the ultrafiltration membranes, when they are simultaneously subjected to transmembrane pressure and tangential flow. To fulfill this challenge a new tangential ultrafiltration SAXS cell has been developed at the "Laboratoire de Rhéologie". This cell has allowed to combine SAXS at the European Synchrotron Radiation Facility (ID02 beamline) with membrane separation (Fig. 1). It has been performed under various crossflow and permeation conditions. The high beam collimation provided by the modern synchrotron source makes x-ray scattering an ideal tool for this work [2]. Systems studied are anisotropic colloidal aqueous dispersions composed of sepiolite rods 1 micrometer long and 0.01 micrometers in diameter, a natural non-swelling fibrous clay from Vallescas, Spain. The initial concentration of the filtered suspensions C0 = 0.01 g/ml (0.5 vol%), is in the semi-dilute domain for which the suspensions exhibit a shear thinning rheological behavior. The tangential ultrafiltration SAXS cell used in these measurements contained a flat polysulfone ultrafiltration membrane (100 kD). During filtration, a transmembrane pressure P = 1.105 Pa is applied and successives tangential flows Q were imposed. The permeation flux J is continuously measured and simultaneously the x-ray beam (40 x 250 micrometer) crossed the lateral dimension of the cell to probe the structure of the concentration polarization layer during time t at different heights z above the membrane (Figs. 1 and 2). As already described in previous studies concerning frontal filtration mode [3-4], a calibration curve relating the absolute scattering intensity to the particles concentration has allowed to deduce the concentration profile as a function of the distance z to the membrane. In Fig. 2, a transmembrane pressure of 1.105 Pa was applied at t = 12 min. The lower retentate tangential flow Q (0.03 L/min) beginning t = 24 min initiated the growth of the deposit layer. With successive higher applied Q, a continuous increase in thickness, anisotropy and concentration of the deposit layer was observed with a simultaneously gradual decrease of the permeation flux with time. The high level of concentration reached in the deposit, up to 50 times the initial concentration, and the highly anisotropic structure formed, would certainly play a decisive role in the mechanisms which control the filtration performance of the sepiolite dispersions. The success of these first measurements on a well known colloidal system open new tremendous opportunities to study in-situ the mechanisms occurring in cross flow filtration. It would bring essential experimental data necessary for improvement in theoretical and numerical modelling. Fig. 2: Permeation flux J(t), concentration profile and SAXS pattern versus time and distance z to the membrane, deduced from in-situ SAXS during ultrafiltration of sepiolite suspensions at initial concentration C0 = 0.01 g/ml, for various pressure and cross flow conditions. Keywords: colloid, anisotropic, crossflow ultrafiltration, deposition, structure, small-angle x-ray scattering. [1] J.C. Chen, Q. Li, and M. Elimelech, Adv. Colloid Interface Sci., 107, 83 (2004). [2] T. Narayanan, O. Diat, and P. Bosecke, Nucl. Instrum. Meth. A, 467, 1005 (2001). [3] F. Pignon, A. Alemdar, A. Magnin, and T. Narayanan, Langmuir, 19, 8638 (2003). [4] F. Pignon, G. Belina, T. Narayanan, X. Paubel, A. Magnin and G. Gésan-Guiziou, Journal of Chemical Physics, 121(16), 8138 (2004).