Simulation of monomeric material transport in reactive extrusion revealed that two different situations can be distinguished in the partially filled zone of a twin-screw extruder. Material is transported from the bottom of: the barrel to the top of the screw by drag or material remains on the bottom of the channel because of gravity. Which process takes place can be predicted when the ratio between the Froude number (gravitation forces) and the Reynolds number (viscous forces) is known. From experiments in a perspex model of a corotating, self-wiping, twin-screw extruder, using glycerol and silicone oil as model fluids, it could be observed which situation can be expected at different filling degrees, screw speeds, throughputs, and viscosities. In the case of a constant filling degree, material moves up with the screw flight at a certain value of Fr/Re. At higher Fr/Re ratios, a larger filling degree is needed for the screws to drag material up with the screw flights. When the filling degree reaches one-half, material moves up with both the pushing and the pulling screw flight. On the basis of these observations, a model is proposed to determine the backflow in the partially filled zone of a corotating, self-wiping, twin-screw extruder, dependent on viscosity and screw speed.