Abstract The phase behaviour of pure casein micelle/κ-carrageenan/water systems has been studied using turbidimetry, microscopy and rheology. The phase state diagram of these systems has been established at 5 and 60°C and the mechanisms are discussed. There is a significant difference between the phase diagram at 5°C and at 60°C in relation to the carrageenan concentration. At a temperature of 5°C, the data demonstrate that, depending on the relative amount of each component, the system behaves like a liquid, which can be either stable or unstable. Polymer adsorption on casein micelles at low κ-carrageenan concentration is suggested. This phenomenon is followed by depletion at higher concentrations. Interestingly, this later instability occurs at a very low concentration of κ-carrageenan (∼0.01%, w/w). As the κ-carrageenan concentration increases, the system gets trapped into a gel state. It is proposed that a casein/κ-carrageenan composite network is formed in which κ-carrageenan molecules act as bridges between casein particles. At a temperature of 60°C, when carrageenan chains adopt a coil conformation, depletion flocculation occurs when the concentration of carrageenan is around 0.3% (w/w).