Studies on dynamic mechanical analyses, infrared spectroscopy and wide angle X-ray diffraction show that degree of crystallinity of maleated EPDM rubber with 75 % ethylene content decreases on neutralization by zinc oxide-stearic acid when an ionic elastomer is produced, Incorporation of zinc stearate which acts as an ionic plasticizer at the processing temperature, and precipitated silica as a filler cause a further reduction in the extent of crystallinity.