In this paper, we have studied the residence time distribution (RTD) in a modular Co-kneader. The Co-kneader is a special modular single-screw extruder with simultaneous rotating and axial movements of the screw. In addition, the thread is interrupted, and fits with the pins along the barrel inducing a self-wiping action. This type of machine is commonly named a reciprocating single-screw extruder. It provides good mixing at low temperature with short residence time and therefore, is suitable for thermally sensitive polymers. Several papers have already addressed the co-kneader but there is still a lack of experimental data on RTD. To investigate the RTD, we have used a colored tracer dispersed in polypropylene (PP), that was injected in the flow during the compounding of a PP. The effect of operating parameters such as temperature, feed rate, and screw configuration was investigated, focusing on the influence of mixing and conveying elements. As can be expected, results on various screw configurations show that increasing the number of kneading elements makes the RTD longer. However, for a defined set of elements, their position does not change the experimental RTD. A chemical engineering approach was used to model the RTD, with an equation derived from a cascade of Continuous Stirred Tank Reactor (CSTR). The model allows us to retrieve an elementary RTD for each section and to propose a law for their combination in good agreement with experiments.