In order to understand the effect of bulk matrix morphology on polymer nanocomposite properties, nanocomposites containing chemically similar but morphologically different polyamide matrices and carbon nanofibers were processed and characterized. Two polyamide matrices were used, one amorphous and one semi-crystalline. Experimental results indicated that the reinforcing efficacy of the amorphous matrix was higher than the semi-crystalline matrix at temperatures below the glass transition. At a carbon nanofiber loading of 0.5 wt.%, the experimentally measured modulus with the amorphous matrix exceeded the Halpin-Tsai prediction for an isotropic material. Overall, these results provided distinct evidence that the underlying bulk matrix morphology plays an important role in polymer nanocomposite mechanical design. (C) 2009 Elsevier Ltd. All rights reserved.