The residence time distribution (RTD) of a counter-rotating non-intermeshing twin screw extruder has been studied by a radioactive tracer technique. RTDs were measured at both the die exit and at the first compounding zone of the extruder. The operating parameters investigated were: screw speed, screw flight stagger, and barrel temperature. These parameters have an effect on the mixing efficiency. The mixing efficiency was characterized by fitting measured response data with models comprised of different combinations of ideal reactor elements. The empirical flow models tested were: plug flow and tanks-in-series (with and without recirculation), plug flow and axial dispersion, and the Wolf-Resnick model. The fitted model parameters characterized the observed changes in the RTD due to the variation of the operating parameters.