Abstract The residence time distribution in an intermeshing, co-rotating twin-screw extruder was measured in-line and off-line by stimulus–response technique. FD&C Red 40 was used as the tracer for the study. The die concentration was monitored in-line using a fiber-optic- equipped vis-near Infrared spectrometer while the color concentration in the off-line samples was measured using a charge coupled device (CCD) spectrometer. The in-line results correlated well with the off-line method. The effect of feed rate, water content and temperature on the residence time distribution were precisely determined and fit by the tanks in series (TIS) model.