Thermosetting polyurethane (PU) multiwalled carbon nanotube (MWCNT) nanocomposites at loadings up to 1 wt% were prepared via an addition polymerization reaction. The morphology of the nanocomposites and degree of dispersion of the MWCNTs was studied using a combination of scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and wide angle X-ray diffraction (WAXD), and revealed the nanotubes to be highly dispersed in the PU matrix. Addition of just 0.1 wt % MWCNTs resulted in significant enhancements in stiffness, strength and toughness. Increases in Young's modulus, % elongation at break and ultimate tensile strength of 561, 302 and 397% were measured for the nanocomposites compared to the unfilled PU. The effect of the MWCNTs on the modulus of the PU was evaluated using the Rule of Mixtures, Krenchel and Halpin-Tsai models. Only the Halpin-Tsai model applied to high aspect ratio nanotubes was in good agreement with the modulus values determined experimentally. Strong interfacial shear stress was found between PU chains and nanotubes, up to 439 MPa, calculated using a modified Kelly-Tyson model. Evidence for strong interfacial interactions was obtained from the Raman spectra of both the precursor materials and nanocomposites. When the MWCNTs were added to the isophorone diisocyanate an up-shift of 14 cm(-1) and on average 40 cm(-1) was obtained for the position of the carbon-hydrogen (C-H) out-of plane bending (766 cm(-1)) and isocyanate symmetric stretch (1420 cm(-1)) modes respectively. Moreover, an up-shift of 24 cm(-1) was recorded for the nanotube tangential mode (G-band) for the 1.0 wt % nanocomposite because of the compressive forces of the PU matrix acting on the MWCNTs. The dynamic mechanical (DMA) properties of the PU thermoset and the nanocomposites were measured as a function of temperature. (c) 2007 Wiley Periodicals, Inc.