Nanocomposites were prepared by melt compounding a synthetic isoprene rubber with either a layered clay or a carbon nanotube as the nanofiller. More in particular, a montmorillonite layered clay, either pristine or modified with an ammonium salt as the surfactant, and multiwall carbon nanotubes were used. Composites were prepared with nanofillers as the only filler or in the presence of a remarkable amount of carbon black (60 phr). The formation of a filler network in the composite materials was investigated by studying the Payne Effect and the dependence of the material initial modulus on the nanofiller content. When the organically modified clay was used in the absence of carbon black, a linear increment of the modulus with the clay content was found for an organoclay content up to about 8 phr. Viceversa, two different regimes, at low and high nanofiller content respectively, were observed when the organoclay was used in the presence of carbon black and the shift from one regime to another was calculated to be at about 5 phr as organoclay content. Two regimes were analogously verified in composites with carbon nanotubes added to 60 phr of carbon black.