The co-rotating twin screw extruder is one of the most important processing machines for compounding in the polymer industry. On the pathway from polymerization reactor to final product, a substantial fraction of all thermoplastic materials passes through an extruder at least once. Different extrusion systems can be compared in terms of the residence time distribution (RTD), which depends mainly on screw speed, screw configuration, mass throughput, back pressure, degree of filling, and material properties. The aim of this study was to determine inline the RTD of different elements of a co-rotating twin-screw extruder. The RTD is key to optimizing the mixing ability of an extruder. For both sensitive and reactive materials, it is important to know how long particles remain in the barrel and how long the polymer remains, for instance, in a kneading element. To assess the influence of different screw configurations on the RTD, a low-concentration tracer particle was injected into the feeding section and measured inline by fluorescence spectroscopy both inside the barrel and at the extruder exit. The measurements were conducted using polypropylene with different amounts of organic peroxide. By measuring the residence time at different positions along the screw it is possible to quantify the RTD for just one screw element. Furthermore, we show the influence of different screw configurations on the polydispersity of polypropylene.