The addition of NaCl to milk is known to alter the mineral equilibrium and the structure of casein micelles. The objective of this study was to better understand the light scattering properties and viscosity of milk as a function of NaCl addition (0–500 mmol.L⁻¹), for fresh skim milk and 2× concentrated milk protein. Suspensions (2×) were prepared using osmotic stressing. NaCl was added by dialyzing milk with milk serum (permeate) for 18 h. The presence of NaCl decreased the pH of milk and the zeta potential of casein micelles. When measured under diluted conditions, using dynamic light scattering, the average radius of the casein micelles showed a small increase, from 84 to 88 nm. Total calcium and phosphate in concentrated milk protein suspensions decreased with NaCl concentration, with a corresponding increase of the permeable calcium and phosphate, and of non-sedimentable caseins. In both untreated and concentrated milk protein samples, the viscosity increased with NaCl concentration. The characteristic decay time (τ) and turbidity measured under non-diluted conditions using diffusing wave spectroscopy showed a gradual decrease with NaCl addition. This work demonstrated that the changes to milk with addition of NaCl are mainly caused by the disruption of the internal structure of the casein micelles and changes in the composition of the serum phase, affecting the viscosity of the continuous phase as well as the refractive index contrast.