Quantitative characterization of the state of dispersion and extent of exfoliation is critical in developing processing structure-property relationships in polymer-clay nanocomposites. Quantification of dispersion, exfoliation, and nanostructure in polymer-clay nanocomposites by 3D stereological parameters using image analysis of 2D transmission electron microscopy (TEM) micrographs were recently proposed. The 3D dispersion quantifying parameters are designed such that they are free of the bias associated with not sampling the true particle diameter in a 2D TEM section. In this article, the ability of the proposed 3D dispersion quantifying parameters to describe the dispersion over the entire possible range of exfoliation, and to capture independent aspects of dispersion are demonstrated by quantifying several sets of samples that were designed using a polypropylene (PP)/maleated PP/clay system. The details of the image analysis procedure, the underlying challenges, and errors involved in the segmentation process are also discussed. The 3D dispersion quantifying parameters, exfoliation number and inter-particle distance, were critically compared against the standard 2D dispersion quantifying parameters, such as mean length, thickness, and aspect ratio. In all cases examined in this study, the sensitivity and resolution of the 3D parameters in terms of quantifying the dispersion of the nanostructure appeared comparable if not better than the standard 2D parameters. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 396-411, 2011