The reinforcement and concomitant nonlinear viscoelastic behavior have been investigated for several composites of linear polymer melt with various binary mixtures of nanofillers having different surface chemistries and particle sizes. The dependence of storage modulus and loss factor on dynamic shear strain amplitude has been obtained for several compositions of each binary filler pair. Composites with mixed fillers display nonlinear interactions that are filler pair/matrix specific. Total filler concentration appears to be a major factor, suggesting that nearest neighbor particle spacing is crucial even if the neighbors are of a different filler type. The results are consistent with the theory recently proposed by Sternstein and Zhu in which the high reinforcement by nanofillers is due to the trapping of entanglements and the resultant effects on matrix chain mobility and entropic elasticity. At high strain amplitudes, the storage modulus and loss factor of the composites with binary filler mixtures are found to be nearly independent of the filler mixture ratio, and dependent only on the total filler concentration. A “partial molar” storage modulus for mixed filler composites is defined for future considerations, and specific interactions are presented for two binary filler systems.