This paper describes the phase separation of different lactide and glycolide copolyester solutions, induced by the addition of silicone oil in order to promote protein microencapsulation. The phase diagrams of the ternary CH2Cl2-copolyester-silicone oil systems were established in relation to the composition of the copolyester and the viscosity, i.e. the molecular weight, of the silicone oil. The phase-separated systems were characterized in terms of weight, volume, composition and viscosity of the coacervate and supernatant. The effect of the nature of the coating polymer and the molecular weight of the coacervation agent (silicone oil) on the characteristics of the phase-separated system is discussed.