The mean residence time (MRT) and the residence time distribution (RTD) of polypropylene in a twin-screw extruder was determined directly in-line with the help of near infrared (NIR) spectroscopy and the use of an UV-absorber as tracer. Different experiment alignments such as screw speed, mass throughput, back pressure as well as tracer content and their influence on MRT and RTD have been investigated. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014, 131, 39919.