Two likely causes of Type C Damping in highly entangled polymers are interfacial slip and shear banding. To isolate these mechanisms, we use confocal microscopy and particle image velocimetry to visualize flow in a planar-Couette shear. Polybutadiene (Mw=200K, 1.1 M) solutions with different entanglement densities (8≤Z≤56) are sheared in narrow gaps ∼35μm. Not only does the velocity at the boundaries violate the no-slip condition, but the velocity profiles are linear. This is inconsistent with shear banding. The measured shear rates and stresses are used to characterize interfacial slip. © 2008 American Institute of Physics.