The stability of corn oil-in-water emulsions coated by milk proteins, sodium caseinate (CAS), or whey protein concentrate (WPC), was compared under the environmental stress of pH change. Emulsions were prepared at 0.1 of protein:oil because the majority of droplets were relatively small ($d_{32}=0.34$ and $0.35;{mu}m$, $d_{43}=0.65$ and $0.37;{mu}m$ for CAS- and WPC-emulsions, respectively) and there was no evidence of depletion flocculation. As the pH of the emulsions was gradually dropped from 7 to 3, there was no significant difference in the electrical charges of the emulsion droplets between the 2 types of emulsions. However, laser diffraction measurements, microscopy measurements, and creaming stability test indicated that WPC-emulsions were more stable to droplet aggregation than CAS-emulsions under the same circumstance of pH change. It implies that factors other than electrostatic repulsion should contribute to the different magnitude of response to pH change.