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Blends of ethylene-propylene-diene (EPDM) and of metalneutralized, sulfonated EPDM elastomers with polypropylene were prepared, and their structure-property relationships were investigated. Extensive rheological measurements, including linear viscoelastic dynamic mechanical studies, were conducted on the blends. As might have been expected, the properties were a strong function of the blend compositions. Both blend systems exhibited multiple transitions indicative of molecular incompatibility of the blend components. These observations are consistent with the morphologies that were observed by scanning electron microscopy using selective extraction techniques. Both blend systems revealed two continuous interwoven phases consisting of elastomer and polypropylene. However, on a relative basis, the moduli and flow of the two blend systems differed significantly; blends of sulfonated EPDM possess higher moduli and lower flow values. The physical and flow property differences between the two systems are directly attributed to the difference in the structure of the elastomer components. Porter (7), and Ho and Salovey (8) found that, at certain blending ratios, both components (which were not miscible) exhibited continuous interwoven phases similar to the interpenetrating systems described by Frisch and Klempner (9), by Sperling and coworkers (10, ll), and by others as reviewed in the text by Manson and Sperling (3). The characteristic size of the continuous phases in TPO is larger, however, being in the order of one p m or higher. Some very recent work done in this area by E. Martuscelli and others is described in a manuscript by Martuscelli, Palumbo, and Kryszewski (12). Improvements in the elastic properties of TPE blends can be realized by using an elastomeric component that would show improved elastic behavior along with a capability for thermoplastic processing. As noted above, W. K. Fischer (6) tried to achieve this by partially crosslinking the elastomer chemically and, more recently,Coran and Pate1 (13) advocated what seems to be a high