Covulcanization of elastomer blends constituting styrene-butadiene rubber (SBR) and ethylene-propylene-diene (monomer) rubber (EPDM) was successfully performed in the presence of reinforcing fillers like carbon black and silica by using a multifunctional rubber additive, bis(diisopropyl)thiophosphoryl disulfide (DIPDIS). The polarity of EPDM rubber was increased by a two-stage vulcanization technique, which allowed the formation of rubber-bound intermediates. In this way the migration of both curatives and reinforcing fillers in the EPDM-SBR blend could be controlled and cure rate mismatch could be minimized. The process significantly improved the physical properties of the blend vulcanizates. The phase morphology, as evident from the SEM micrographs, was indicative of the presence of a much more compact and coherent rubber matrix in the two-stage vulcanizates. Different accelerator systems were studied to understand better the function and effectiveness of DIPDIS in developing homogeneity in the blends of dissimilar elastomers. (C) 2004 Wiley Periodicals, Inc.