A flow visualization study is reported for the motions of a mineral oil and a thermoplastic polymer individually in a modular co-rotating twin screw extruder. Flow in particular elements and combinations of elements is described. Experiments were also carried out to determine the residence time distribution. The influence of modular screw configuration and screw speed on residence time distribution has been determined. It was found that replacing screw elements with kneading discs or left handed screw elements increases residence time and broadens the residence time distribution. Increasing screw speed at fixed feed rate reduces both the mean residence time and the variance sigma2 of the residence time distribution. Increasing feed rate at fixed screw speed has a similar effect. The variation of sigma2 with position along the screw axis was measured. The results are interpreted in terms of the fluid mechanics of twin screw extruders including the Akro-Co-Twin Screw program.