In the interfacial polymerization of poly(urea-urethane) membranes that result in the formation of microcapsules, the mechanism of microencapsulation can be complex. To clarify this process, a series of organic solvents with different degrees of compatibility between the solvent and the polymer, consisting of diethyl phthalate (DEP), dibutyl phthalate (DBP), and dioctyl phthalate (DOP), was used to study the polymerization of triisocyanate in both film and microcapsule formation. The physical properties of both the films and of the microcapsules were then determined by using a combination of differential scanning calorimetry, optical microscopy, and atomic force microscopy. The glass. transition temperatures of films with limiting amounts of DEP, DBP, and DOP and without the organic solvent were, respectively, 98, 115, 123, and 124 degreesC. The triisocyanate + DOP system had a lower critical solution temperature of about 5 degreesC, with a two-phase domain structure for the membrane synthesized with DOP above the critical solution temperature. The shape of the microcapsules was found to be spherical, biconcave, and biconcave with,a hump for compositions with weight fraction of DOP, w(s), being 0, 0.3-0.5, and 0.78, respectively. By taking into account the compatibility between the solvent and the polymer, the combined results suggest that chemical cross-linking, swelling, and phase separation can be coupled to control the mechanism in microencapsulation of poly(urea-urethane) membranes to form microcapsules. A homogeneous elastic membrane is formed with organic solvents that are compatible with the wall-forming monomers, whereas the membrane becomes brittle and heterogeneous with less compatible solvents due to phase separations.