Skim milk filtration is performed either at low or high temperature. However, there is still a lack of knowledge concerning the influence of temperature on membrane fouling. We used in situ small-angle X-ray scattering (SAXS) to study external membrane fouling during crossflow ultrafiltration of milk protein (casein micelle) dispersions at 12 • C and 42 • C. Casein micelle concentration distribution was measured in the concentration polarization layer and the deposit in a three-step filtration experiment that consisted of (i) fouling development step (with applied pressure and average cross-flow velocity fixed at 110 kPa and 3.1 cm s − 1 , respectively), (ii) pressure relaxation step (with pressure reduced to 10 kPa), and (iii) deposit erosion step (with crossflow velocity increased to 15.6 cm s − 1). Despite a higher average filtrate flux obtained at 42 • C, filtration at 42 • C resulted in the formation of a thicker and more concentrated deposit than filtration at 12 • C. At both temperatures studied, subsequent pressure relaxation resulted in deposit swelling, which was more intense in the less-compressed external part of the deposit. Deposit swelling rate was significantly higher at 42 • C than at 12 • C. The swelled deposit obtained at 42 • C was more eroded by the crossflow compared to the poorly swelled deposit obtained at 12 •. This suggests that deposit formation and compression were more reversible at 42 • C than at 12 • C.