The structure property behavior of an ethylene-propylenediene ionic terpolymer containing varying amounts of ZnSt was investigated. Studies show that ZnSt acts as a reinforcing filler under ambient conditions. Stress-strain behavior, percent relaxation, hysteresis and permanent set were all found to increase with increasing ZnSt content. Increases in stress values upon annealing for all samples were attributed to increased ionic domain formation and growth. Aging was found to alter stress values at elongations beyond the elastic region. SAXS data indicates that structural rearrangement occurs with aging. DSC studies confirmed the effects of aging in that a difference occurred in thermal behavior between 1st and 2nd meltings. DSC also indicated the presence of an impurity, which is believed to be stearic acid. Water absorption studies conducted on the aged systems suggested that ZnSt either interacts with the Zn salt group tying up absorption sites or diffuses towards Zn salt groups as possible nucleation sites and forms a water barrier. Water negatively affects ZnSt's ability to act as a reinforcing filler. A rise in SOC values indicated non-Gaussian behavior for all samples. WAXS indicates that a portion of the ZnSt crystals hold their orientation upon stress release. The ZnSt loaded and unloaded systems had distinctly different morphologies. Both SEM and WAXS studies of the 303-50 sample indicated small ZnSt particles of less than 400°A in size. Comparison with modulus studies of other hard phase systems lent major support to the postulation of very small ZnSt crystals. / Master of Science