Nanocomposites of high-density-polyethylene (HDPE) and organically (dimethyldioctadecylammonium) modified montmorillonite (OM) were prepared and the effect of non-ionic surfactants on the OM exfoliation and composite properties (tensile + gas-permeation) was studied. Amphiphilic block and random copolymers of different chemical structures were used as dispersing agents. The presence of copolymers in the composites led to polymer intercalation that increased the d-spacing and facilitated the exfoliation. Consequently, the permeability coefficient (oxygen) of the nanocomposites was decreased and their stiffness increased. End-functionalized oligomers proved to be more efficient in dispersing the OM than copolymers in which the polar units are randomly distributed along the polymer chain. Poly(ethylene-co-vinyl alcohol) increased the d-spacing but did not improve the properties of the composite probably due to 'bridging' the silicate layers, which hindered the exfoliation. The OM exfoliation could be enhanced to such an extent that an inclusions' average aspect ratio of 150 was estimated from the oxygen-permeation measurements. With increasing exfoliation, the stiffness, strength and gas-barrier properties of the composites improved significantly. The oxygen permeability of the HDPE nanocomposites was cut to less than half, thus offering a strong barrier to oxygen and humidity useful for food and drug packaging. (c) 2005 Elsevier Ltd. All rights reserved.