Precipitated silica improves the physical properties of a zinc-sulfonated ethylene-propylene-diene terpolymer (SEPDM) of high (i.e. 75 wt%) ethylene content. The properties studied include hardness, stress-strain characteristics, tear strength, abrasion resistance and hysteresis. Processability studies carried out by using the Monsanto Processability Tester (MPT) show that silica increases the melt viscosity of the zinc-sulfonated EPDM. Results obtained from dynamic mechanical analysis (d.m.a.) show that silica reinforces the backbone chain and weakens the ionic associations. The results obtained from dielectric thermal analysis (d.e.t.a.) confirm the d.m.a. results and suggest that the formation of multiplets and clusters present in the SEPDM are adversely affected by silica.