This paper describes transient experiments in an industrial twin-screw extruder reactor used for the finishing stage of nylon-6,6 polycondensation. Transient experiments were specially designed to obtain pertinent information from the industrial extruder, such as the average residence time and the degree of filling. A transient model for this process was developed. The extruder is modeled as two compartments, the partially filled degassing zone where water is removed from the polymer and the fully filled zone where the polycondensation and degradation reactions take place. The axial dispersion model is successfully applied to explain the results of the experiments done in the industrial plant.