Mixtures of globular protein (beta -lactoglobulin) aggregates and polysaccharides (kappa -carrageenan) were studied using dynamic and static light scattering, size exclusion chromatography, and optical microscopy. Above a critical K-carrageenan concentration the protein aggregates phase separate and form spherical microdomains, which slowly sediment. The effect of concentration and size of the protein aggregates was investigated. The phase separation is initially reversible upon dilution, but the microdomains become progressively more stable until after several days they can no longer be redissolved.