The imidization of poly(styrene-co-maleic anhydride) (SMA) with aniline by reactive extrusion is investigated in a co-rotating twin-screw extruder. During this reactive extrusion, the process temperature is much higher than the boiling point of the aniline. Accordingly most of the aniline, should be vaporized immediately after being fed into the extruder, occupies the unfilled part of the extruder, and is transferred to the melt phase where it is consumed through the reaction. Based on the mechanism of this vapor-melt heterogeneous process and experimental data for residence time distribution (RTD) in the extruder, a continuous process model is developed. The effects of operating conditions including temperature, throughput, and screw rotation speed on the reaction kinetics are discussed by both experimental data and model simulation. The results indicate that the residence time and the mass transport of aniline from vapor to melt phase should play significant roles in this heterogeneous reactive extrusion process. (C) 2009 Elsevier Ltd. All rights reserved.