Ionomers prepared by the sulfonation of ethylene propylene diene (EPDM) rubber have attractive properties for applications such as thermoplastic elastomers. The sulfonation reaction is usually carried out in solution by using volatile organic compounds (VOCs), which adds cost to the product and is environmentally unfriendly. The melt sulfonation of EPDM has been attempted in the past, but the results were not very satisfying. However, most of that work was done in the early 1980s and with single screw extruders or batch mixers. This paper discusses the melt sulfonation of EPDM by reactive extrusion using a twin screw extruder (TSE). Zinc stearate was added as a processing aid, but it is also an integral part of the formulation of a useful sulfonated EPDM thermoplastic elastomer. The addition of zinc stearate improves the processability of the ionomer and also improves its mechanical properties. The details of the extrusion reaction and the structure and properties of the ionomers will be discussed.