The aim of this work is to understand the influence of operating conditions (time, transmembrane pressure (TMP), cross-flow velocity (v)) on the casein micelles fouling layer properties (build-up, sol-gel transition and reversibility behaviour during TMP relaxation) during cross-flow filtration. Small-Angle X-Ray Scattering (SAXS) has been coupled to cross-flow ultrafiltration to determine the behaviour of casein micelles concentrated layers at the membrane surface. Concentration profiles have been deduced from the scattering intensities registered in-situ in the course of the cross-flow ultrafiltration of casein micelles dispersions. SAXS and filtration results for a reference experiment (TMP = 110 kPa, v = 0.03 m∙s−1, 2h30 of filtration, 25°C) show the formation of a thick fouling layer of 275 μm and reaching 307 g∙L−1 at the membrane surface, which is above the sol-gel transition of casein micelles (≈180 g∙L−1). Furthermore, results show some reversibility of the outer fraction of fouling layer and irreversibility of its fraction close to the membrane surface. Those results are important for understanding the concentrated layer build-up of casein micelles during cross-flow ultrafiltration but more originally the TMP relaxation behavior of casein micelles fouling layer linked to different operating conditions