A modeling study for the polymer flow and mixing in a corotating twin-screw extruder is presented. The residence time distribution (RTD) in a fully intermeshing corotating twin-screw extruder was measured, using iron powder as tracer, and an LDPE as flow material. Pulse-type input signal experiments were performed in different working conditions. To fit the experimental data obtained on two different screw profiles, the one- and two-parameter flow models described in literature were tested. The best fitting of the experimental RTD curves was obtained by using a delayed tanks in series model with internal recirculation, or a delayed axial dispersion model. This is consistent with the physical characteristics of the flow in a fully intermeshing corotating twin-screw extruder.