The present investigation is an experimental study of the mixing of two phases of strongly different viscosity in a co-rotating twin-screw extruder (TSE). Two cases were considered: a miscible (copolymer of ethylene and vinyl acetate/bis(2-ethylhexyl)phtalate [EVA/DOP]) and an immiscible (EVA/ethylene-glycol [EVA/EG]) binary system. The residence time distribution (RTD) has been determined for each phase of both systems for different screw profiles and different operating conditions. The behavior of the first system is simple: the type of flow is basically the same. However, in the case of nonmiscibility, when the flow rate is high, the shape of the RTD of both phases may be different. We interpret this result with the appearance of a lubrication phenomenon. (c) 2005 Society of Plastics Engineers.