This study focuses on bringing a better understanding of the mechanisms involved in the formation of the concentration polarization and fouling phenomena during the cross-flow ultrafiltration process of colloidal Laponite clay suspensions. New cross-flow ultrafiltration cells were designed to perform firstly time resolved in-situ small angle X-ray scattering (SAXS) and secondly time resolved in-situ micro particle image velocimetry (micro-PIV) at the vicinity of the membrane surface during the filtration. These two methods have allowed to access to the concentration profiles and the velocity field as a function of the distance z from the membrane surface with respectively 20 μm and 30 μm accuracy, within the concentration polarization layers. The results obtained show an increase of the volume fraction related to a decrease of the velocity within the layers formed during the process. The correlation of these results with the rheological behavior of the suspensions permitted to access to the calculated stress field within the concentrated layers during the cross-flow ultrafiltration process. Three different regions near the membrane surface have been emphasized with different shear stress/shear rate behaviors. The important role of the rheological behavior and inter particle interaction in the dynamic evolution in space and time of the accumulated layers has been highlighted.