Casein-casein interactions can be very useful for tuning structural and physical properties of mixed casein micelles for food and biotechnology applications, particularly in the dairy industry. The present work focuses on the structures of mixed micelles formed by stepwise addition of a micellar β-casein solution to either 0.05 or 0.3 mM κ-casein, i.e. below or above the critical micellar concentration (CMC) of κ-casein, respectively. Insight into the thermodynamics of this mixed micellization was achieved using isothermal titration calorimetry (ITC), which indicated a less favorable enthalpy of formation and a downward shift of the CMC of mixed micelles with 0.3 mM κ-casein. Hydrophobic interactions are the main driving force behind the mixed micellization as probed by using pyrene, a hydrophobic fluorophore. Structural characterization by dynamic light scattering (DLS) and small angle X-ray scattering (SAXS) revealed that addition of micellar β-casein to 0.05 mM κ-casein oligomers caused strong perturbation and formation of smaller micelles compared to when added to 0.3 mM κ-casein in micellar state. Thus, association of β-casein with κ-casein in these two different physical forms of loosely interacting oligomers and micelles, respectively, can modulate formation of mixed micelles with regard to structural properties such as shape, size and molecular packing (compactness).