The influence of the melt-blending conditions on the structural, rheological, and interfacial properties of modified montmorillonite/Polyamide-12 nanocomposites has been studied performing transmission electron microscopy observation combined with X-Ray diffraction and rheological experiments. In the dilute regime, for short mixing times, the apparent aspect ratio of primary clay entities, determined from intrinsic viscosity measurements, is shown to increase with rotational speed. At high blade rotational speeds, the viscometric results suggest an almost achieved exfoliation, as confirmed by transmission electron microscopy micrographs. For longer mixing times, a significant drop of viscous dissipation is observed, which is very marked at high blade rotational speeds and attributed to a modification of the particle/matrix interface. In the concentrated regime, the rheological behavior of nanocomposites is attributed to the formation of a network of mesoscopic domains, composed of correlated clay entities. Upon increasing strain during mixing, the clay aggregates within these domains break into intercalated stacks and finally exfoliated layers, as shown by transmission electron microscopy micrographs and wide-angle X-ray diffraction patterns. The melt state elastic and viscous properties of the nanocomposites are mainly governed by the networked domains, and not by the nature and properties of the structure within the domains. POLYM. ENG. SCI. 46:986–994, 2006. © 2006 Society of Plastics Engineers.