The viscosity of uncompatibilized polymer blends often shows a negative deviation from a log-additivity rule at shear rates relevant to processing. This deviation has been attributed to interfacial slip, which is related to the loss of entanglements at the interface. In this work interfacial slip and the effect of reactive compatibilization on this phenomenon are studied in blends consisting of ethylene-propylene rubber and nylon-6. The viscosity and morphology of blends with various compositions and compatibilizer content are investigated systematically. The results indicate that interfacial slip can indeed be important in uncompatibilized systems whereas it is suppressed in compatibilized blends. This is further supported by a study of the viscosity of multilayer structures. Both the effect of reactive compatibilization and of the number of layers on the theology are studied. Again it is demonstrated that reactive compatibilization suppresses interfacial slip.