Aggregation of casein in milk protein concentrates (MPCs) is altered by changes in calcium content. Emulsifying properties of MPCs and stabilities of emulsions formed with MPCs were investigated by examining emulsions formation, adsorption behaviours of proteins and emulsion microstructures. Compared with emulsions formed with higher calcium MPCs at a given protein concentration, emulsions formed with low calcium MPCs were finer, the total surface protein concentration was lower and the protein composition on the surface of the emulsion droplets was altered. Thus, the aggregation state of casein dominates the emulsifying capability of MPC products and the adsorption behaviours of proteins in MPCs. In low-calcium-MPC-stabilized emulsions, the stability of the emulsions decreased with an increase in the emulsion size at low protein concentrations and decreased with increasing protein concentration beyond a maximum level, suggesting that the protein state in low calcium MPCs may cause depletion flocculation in the emulsions.