The effect of the addition of protein particles was investigated on the stability of water-in-water emulsions formed by mixing aqueous dextran and poly (ethylene oxide) solutions. Protein particles with hydrodynamic radii ranging from 15 to 320 nm were produced by heating globular proteins in controlled conditions. The structure of the emulsions was visualized with confocal laser scanning microscopy using different fluorescent probes to label the dextran phase and the protein particles. It is shown that contrary to native proteins, protein particles adsorb at the interface and can form a monolayer that inhibits fusion of emulsion droplets. In this way, water-in-water emulsions could be stabilized for a period of weeks. The effect of the polymer composition and the protein particle size and concentration was investigated.