Nanocomposites were prepared by adding montmorillonite clay to a high-density polyethylene matrix. Their structure, morphology, thermal behavior, and physical- mechanical properties were studied. The filler did not alter much the structure and morphology of the matrix, with the exception of a disruptive effect on the lamellar stacks. The crystallization behavior, equilibrium melting temperature, and work of chain folding of the nanocomposites were also unaltered with respect to that of the PE base polymer. However, significant improvements in physical-mechanical properties were observed. The reason for this increase in performance was ascribed to the interaction between the filler and the matrix, especially because of a reduction in size of the original tactoids to stacks of just a few layers, albeit not intercalated. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 3920-3928, 2009