Composites of polyamide-6 and carbon nanotubes (NT) have been prepared on a corotating twinscrew extruder. It is shown by transmission electron microscopy (TEM) that the nanotubes are dispersed homogeneously in the polyamide-matrix. The electrical conductivity of these composites was analyzed and compared to carbon black filled polyamide-6. It is found that the NT-filled polyamide-6 shows an onset of the electrical conductivity at low filler loadings (4-6 wt%). In agreement with rheological measurements this onset in the conductivity is attributed to a percolation of nanotubes in the insulating matrix polymer. Tensile tests of the NT-composites show a significant increase of 27% in the Young's modulus, however the elongation at break of these materials dramatically decreases due to an embrittlement of the polyamide-6. Blends of these composites and Acrylonitrile/butadiene/styrene (ABS) have been prepared by extrusion. It is shown by TEM measurements that the nanotubes are selectively located in the polyamide-6. These selectively filled polyamide-6/ABS-blends show a highly irregular, cocontinuous morphology. Due to the confinement of the conductive filler to one blend component these materials show an onset in the electrical conductivity at very low filler loadings (2-3 wt%). These findings are explained by a double percolation effect. The NT-filled blends show superior mechanical properties in the tensile tests and in IZOD notched impact tests. (C) 2003 Elsevier Ltd. All rights reserved.