The dispersed phases of a multiphase polymer blend will either form an encapsulation-type phase morphology or the phases will remain separately dispersed, depending on which morphology has the lower free energy. We have developed a model to predict phase morphologies of multiphase polymer blends. Calculations based on the model suggest that interfacial tensions play the major role in establishing the phase structure of a multiphase system, with a less significant role played by the surface areas of the dispersed phases. The model further shows that the phase structure of a multiphase polymer blend can be converted from one type to another by changing the interfacial tensions between one or more pairs of the components using interfacially-active agents such as block or graft copolymers. We have applied the model to different ternary blends of polystyrene, polyethylene, polypropylene and poly(methyl methacrylate), and have compared the predicted morphologies of these blends with experimental results. In each case, the predicted morphologies agree with those found experimentally. In addition, we have successfully converted the phase structures of these blends from one type to another by using interfacially-active block copolymers.