The residence time distribution of wheat flour in a twin screw extruder was determined using NaCl as tracer. Variables studied included screw profile, screw speed water supply, feed supply and die diameter Screw profile, screw speed and feed supply strongly affected the mean residence time and the flow pattern in the extruder The effect of the screw profile and screw speed was more pronounced than that of feed supply. With an ISIM simulation software pack-age, a residence time distribution model was developed.