Nanocomposites of styrene and vinyl phenol copolymers, which contain varying (10, 20, and 40 mol %) vinyl phenol content, were prepared with 1 wt % unoxidized, 1 wt % oxidized, and 5 wt % oxidized carbon nanofibers. Dynamic mechanical analysis and differential scanning calorimetry indicate that the composites prepared from oxidized nanofibers exhibit improved thermal and structural properties relative to those prepared from unoxidized nanofibers. The optimum enhancement in the mechanical and thermal properties was observed for the composite containing oxidized nanofibers and the 20% vinyl phenol copolymer. These results are in excellent agreement with our previous work on carbon nanotube-polymer composites and suggest that the presence of intermolecular interactions between the copolymer matrix and nanofibers are responsible for the observed property enhancement. 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3053-3061, 2006.