Skim milk microfiltration MF is widely applied for separation and concentration of serum proteins (SP) and casein micelles (CM). The weight ratio between SP and CM (R) as well as their concentrations in the system (cSP and cCM) change during MF. Therefore, optimization of MF requires knowing of pertinent material properties (osmotic pressure π, apparent viscosity µ, and sol-gel transition concentration csg) of mixed SP and CM dispersions having different R, cSP, and cCM. Our study focuses on the role of SP in the compressibility and rheological behavior of mixed SP/CM dispersions and its significance for milk MF, that are rarely discussed in the literature. Powders of SP and CM were dispersed in their natural environment: permeate of skim milk ultrafiltration. Compressibility of mixed dispersions was measured via the osmotic compression technique. Rheological properties were studied via the steady shear and oscillatory tests. All measurements were done at 20°C in a wide range of total proteins concentration and different values of R. For a given cCM, increase of R in mixed SP/CM dispersions (a) increased π (decreased the compressibility), (b) increased µ, (c) shifted the onset of non-Newtonian behavior to higher cCM, (d) lowered the yield stress. Oscillation measurements revealed that csg shifted to higher total proteins concentration as R increased. At the same time, the relative viscosity of mixed dispersions decreased and flowability improved as R increased. Relatively small serum proteins (a) space larger casein micelle particles in the system and (b) lower micelle-micelle interaction (serving as a dispersant for casein micelles). This results in significant changes of material properties mixed SP/CM dispersions that must be accounted in the modelling and optimization of skim milk concentration-separation via MF (as well as other milk concentration processes).