Phase separation, as a result of thermodynamic incompatibility between casein micelles and added polysaccharide, occurs when polysaccharide gums are added to milk based products. Addition of [kappa]-carrageenan prevents the undesirable phenomenon, however, the mechanism by which this is achieved is not fully known. The existing theories, although reasonable, fail to fully explain [kappa]-carrageenan's 'milk reactivity'. Therefore, the purpose of this research was to analyze the ability of [kappa]-carrageeenan to prevent phase separation under a variety of conditions and to provide a viable explanation. Field emission scanning electron micrographs, dynamic light scattering and micro differential scanning calorimetry results show that interaction occurs between casein micelles and [kappa]-carrageenan. Dynamic light scattering and phase separation experiments also show that interaction is necessary but not sufficient to prevent phase separation and further suggest that aggregation of [kappa]-carrgeenan helices is also required. These results, coupled with results from previous projects in our lab, lead to a mechanism that involves both casein-[kappa]-carrageenan and [kappa]-carrageenan-[kappa]-carrageenan interactions, which prevents phase separation.