The determination of the local residence time distribution (RTD) in a co-rotating twin screw extruder was performed using a UV tracer method. This method was used to test the RTD response to a pulse input into the different elementary screw elements such as right- and left-handed screw elements and mixing elements. Experimental RTD data were used in order to compare the local dynamic response of the basic screw elements. These results were qualitatively discussed on the basis of flow pattern developed in the pumping section of the extruder. On the other hand, predictions of the RTD functions were attempted from classical chemical engineering models such as axial dispersion model and ideal stirred tanks in series model. Although these models predict quite well the shape of the RTD functions, the tail of the distribution was not correctly described. Thus, from these basic models, a new concept of residence time spectrum was introduced.