The effect of onium-ion exchanged montmorillonite clay (organoclay) on the rheology and physical properties of maleated ethylene-octene copolymer and ethylene-propylene rubber elastomers has been examined. The formation of a nanocomposite hybrid, containing substantial amounts of exfoliated clay, was accompanied by significant increases in the complex viscosity, elastic and loss moduli, and a narrowing of the region of linear viscoelasticity, attributed to both filler/polymer and filler/filler interactions. The properties of blends of these elastomers with polypropylene were dominated by the reinforced elastomeric matrix, into which the organoclay resided preferentially. The nanocomposite blends exhibited very fine morphologies, as well as a good balance of stiffness and ductility and enhanced thermal stability.