Skim milk powder (SMP) and milk protein concentrate (MPC) are evaporated before spray drying. It would be an economical advantage to obtain a solution of higher % total solids (TS) before spray drying. This is problematic because it leads to an increase in the viscosity. Ultrasound or sonication has been shown to decrease the viscosity of solutions, therefore, this research studied the effects of sonication on the viscosity of reconstituted MPC (rMPC) and SMP (rSMP) as influenced by %TS at 60°C in a continuous operation. MPC and SMP were reconstituted to 30–34% TS and 46–54% TS, respectively and circulated in a continuous operation at a flow rate of 1.8 L/min for a total of 60 min and 15 min respectively before being sonicated (Hielscher UIP500 sonicator with flow cell). Samples were sonicated (70% amplitude) for a total of 6 min (samples collected after every 2 min). The viscosity was measured at 60°C using a viscometer. Statistical analysis was performed on triplicates using t-tests (α = 0.05). Overall, there was an increase in viscosity with an increase in solids content and a decrease in viscosity upon sonication for both rSMP and rMPC. For rSMP, as compared with presonication. the decrease in viscosity after 2, 4, and 6 min of sonication was 25.3, 29.8, and 33.0% (for 46% TS); 16.0, 37.9, and 42.0% (for 50%TS); 5.7, 9.6, and 13.3% (for 52% TS); 12.0, 16.2, and 22.6% (for 54%TS), respectively. For rMPC, as compared with presonication, the decrease in viscosity after 2, 4, and 6 min of sonication was 30.6, 36.6, and 46.8% (for 30% TS), 19.5, 30.3, and 36.0% (for 32% TS), 24.4,19.2, and 25.0% (for 34%TS), respectively. Sonication significantly decreased the viscosity of rMPC and rSMP at 2, 4, and 6 min as compared with presonication. For rMPC, the mean viscosity of the 34% TS sample after 6 min of sonication was lower than the mean viscosity of 30% TS sample before sonication. Thus, allowing for an increase in TS by 4% to be spray dried without increasing the viscosity of the solution. For rSMP, sonication did not allow for an increase in %TS without increasing the viscosity of the sample, which can be attributed to the age thickening of the samples