A series of alkyl ammonium/MMT organoclays were carefully selected to explore structure-property relationships for thermoplastic polyurethane (TPU) nanocomposites prepared by melt processing. Each organoclay was melt-blended with a medium-hardness, ester-based TPU, while a more limited number of organoclays was blended with a high-hardness, ether-based TPU. Wide-angle X-ray scattering, transmission electron microscopy, particle analysis, and stress-strain behavior were used to examine the effects of organoclay structure and TPU chemical structure on morphology and mechanical proper-ties. Specifically, the following were observed: (a) one long alkyl tail on the ammonium ion rather than two, (b) hydroxy ethyl groups on the amine rather than methyl groups, and (c) a longer alkyl tail as opposed to a shorter one leads to higher clay dispersion and stiffness for medium-hardness TPU nanocomposites. Overall, the organoclay containing hydroxy ethyl functional groups produces the best dispersion of organoclay particles and the highest matrix reinforcement, while the one containing two alkyl tails produces the poorest. The two TPU's exhibit similar trends with regard to the effect of organoclay structure. The high-hardness TPU nanocomposites showed a slightly higher number of particles and clay dispersion. The organoclay structure trends are analogous to what has been observed for nylon 6-based nanocomposites; this suggests that polar polymers like polyamides, and apparently polyurethanes, have a relatively good affinity for the polar clay surface; and in the case of polyurethanes, the high affinity of the matrix for the hydroxy ethyl functional groups in the organoclay aids clay dispersion and exfoliation. (c) 2006 Elsevier Ltd. All rights reserved.