Studies that use only rheological data to deduce the slip velocity between immiscible polymer melts usually involve applying shear deformation to a stack of parallel multilayers. The maximum stress at which the slip velocity can be measured in these experiments is limited by the distortion of the parallel layers. However, polymer extrusion involves flow at significantly higher stresses than that accessible in these experiments. In order to understand slip in the context of extrusion, we have investigated slip at the interface between two immiscible polymer melts undergoing pressure-driven capillary flow. To enable the measurement of slip velocity at the polymer/polymer interface we have adapted the Mooney method, a method originally developed to study wall slip. Using the method, we measured the dependence of the interfacial slip velocity on the interfacial shear stress for cylindrical core-sheath samples of Polypropylene and Polystyrene. In agreement with prior work on multilayer sandwiches, we found two distinct power-law regimes in the relationship between the interfacial slip velocity and the interfacial shear stress. The power-law exponent changes from a value of approximately 3 at low shear stress values to approximately 2 at high shear stresses. We then investigated the effect of varying the temperature on the slip velocity and explore the consequences of time-temperature superposition for the two polymers. Interestingly, the interfacial slip velocities can be superposed using the WLF parameters of one of the polymers but not the other. Finally, I will discuss the connection between interfacial slip and the roughness observed at the polymer/polymer interface.