Interfacial slip between high density polyethylene (HDPE) and polystyrene (PS) melts was studied. The HDPE/PS layered structure was subjected to steady shear in a Cambridge shearing system. Through thickness melt velocities of the HDPE/PS layered structure were measured in situ by a confocal microscope. Velocity profiles of both HDPE and PS layers were obtained. Velocity discontinuity was observed at the HDPE/PS interface from the velocity profiles and indicated interfacial slip. The slip velocity obtained from velocity discontinuity is in good agreement with that deduced by independent rheological measurements. (C) 2003 The Society of Rheology.