Rheological measurements are used to compare clay nanocomposites prepared through melt mixing using two different polypropylene matrices. Steady state and transient nonlinear theological experiments are employed to separate the contributions of flow induced orientation of the tactoids and particulate network build-up. The conditions under which the theological properties are dominated by the aggregate network are subsequently identified. Under these conditions, the low frequency linear viscoelastic behavior is analyzed using scaling concepts for fractal networks to determine the degree of network formation by exfoliation. Moreover, the high frequency behavior of the moduli can be used to quickly assess the dispersion quality. The results from the analysis of the linear viscoelastic data are compared to structural features extracted from electron microscopy and small angle X-ray scattering data. (c) 2007 The Society of Rheology.