The transesterification of ethylene-vinyl acetate (EVA) copolymer to produce ethylene-vinyl alcohol copolymer (EVAL) was studied in both a batch reactor and in a modular corotating twin screw extruder with various screw configurations. Increasing the number of left-handed screw elements and kneading disc blocks increases the extent of conversion under any set of operating conditions. Conversion is increased by decreasing screw speed and reducing throughput. Reactor design models have been developed to represent the modular twin screw extruders. These use a drag flow reactor for screw elements, a continuous stirred tank reactor (CSTR) to represent kneading disc blocks and a plug flow reactor to represent post screw and die regions. Comparisons are made with experiment.