Fluid flow in extruders has been described kinematically using residence time distributions (RTDs). Residence Distributions (RxD), as expressed by Residence Volume Distributions and Residence Revolution Distributions, have shown to collapse the RTD curves onto single master curves based on specific throughput. When the same extruder is used for processing different polymers, differences in RxDs have been attributed mainly to differences in material viscosity, as measured, for example, by the meltflow index. This study explores the role of polymer viscoelasticity in modeling extruder performance. It is shown that viscosity is not the material property that explains differences in RxDs. In one case, two materials with very similar viscosities have very different RxDs and, in a second case, two materials having very different viscosities have very similar RxDs. A calculation of the Weissenberg number for each system shows that viscoelastic effects can underlie the differences in RxD. Strongly elastic fluids tend to travel in a plug-flow-like manner down an extruder and so their RxDs are qualitatively different from those of viscous fluids. The results thus show that comprehensive rheological information is necessary for modeling the flow of polymer melts in extruders. (c) 2006 American Institute of Chemical Engineers