The present work aims to better understand the relation between heat treatment (Ultra High Temperature - UHT), partial demineralization of casein micelles (CMs) dispersions and their corresponded rheological properties. A multiscale analysis (SAXS, solid state NMR and Asymmetrical Flow Field Flow Fractionation (A4F)), has been used. Different samples with different levels of calcium/phosphorous demineralization (up to −30% of calcium content) have been studied before and after UHT treatment. The present data show that CM dispersions, displaying a mild demineralization, display an important decrease in viscosity after UHT. NMR shows that, during UHT, mild demineralized samples display stronger calcium/phosphorus intake than both, the non-demineralized and the strong demineralized ones. SAXS and A4F show that UHT-dependent calcium intake, in mild demineralization dispersions, seems to be responsible for a transformation of large casein aggregates into small and denser ones. This decrease of the large aggregates is probably the main reason of the UHT-dependent decrease in viscosity. These data show that a sample with a rather small amount of demineralization can have a quite important susceptibility to UHT in terms of viscosity.