A new model for the residence-time distribution in a counter rotating twin-screw extruder is compared with experimental results obtained during the gelatinisation of starch and the grafting of polystyrene on starch. The model consists of a transfer function from which the first three moments can be derived and is based on physical principles without any adjustable parameters. In the model and during experiments, parameters like screw rotation, fully filled length and throughput have been varied. By comparing calculated residence-time distributions with the measurements conclusions about the extruder hold-up and the fully filled extruder length can be drawn. As an extra parameter, the density change of the extruded material was needed to model the residence-time distribution curve during the grafting of polystyrene on starch.