Interfacial slip at high-density polyethylene (HDPE)/polystyrene (PS) and high-impact polystyrene (HIPS)/PS interfaces under steady shear was studied. The multilayer structure and energy model for steady shear proposed by Lam and colleagues was employed. Results indicated that there was no interfacial slip at the HIPS/PS interface. However, interfacial slip was detected for HDPE/PS under steady shear. Small interfacial thickness and weak interactions between HDPE and PS was proposed as the reason for interfacial slip at the HDPE/PS interface. Chain orientation under shear was believed to promote chain disentanglement in the interfacial layer and therefore increase interfacial slip. (C) 2003 Wiley Periodicals, Inc.