The effects of vapor-grown carbon nanofiber (VGCNF) weight fraction, high-shear mixing time, and ultrasonication time on the Izod impact strengths of VGCNF/vinyl ester (VE) nanocomposites were studied using a central composite design. A response surface model (RSM) for predicting impact strengths was developed using regression analysis. RSM predictions suggested that an 18% increase in impact strength was possible for nanocomposites containing only 0.170 parts per hundred parts resin (phr) of VGCNFs (similar to 0.1 v%) that were high-shear mixed for 100 min when compared to that of neat VE. In general, the predicted impact strengths increased for high-shear mixing times above 55 min and VGCNF weight fractions below 0.400 phr. The predicted strengths decreased as the VGCNF weight fraction was further increased. Scanning electron micrographs of the nanocomposite fracture surfaces showed that increased impact strength could be directly correlated to better nanofiber dispersion in the matrix. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 128:1070-1080, 2013