Single-wall carbon nanotubes (SWCNT), multiwall carbon nanotubes (MWCNT), and carbon nanofibers (CNF) were oxidized with a conventional H2SO4/HNO3 treatment, and the resulting COOH groups were esterified with phenyl glycidyl ether (PGE). Pristine, oxidized and esterified SWCNT, MWCNT, and CNIF (1 wt%) were dispersed in diglycidyl ether of bisphenol A/polyether triamine, and cured to produce epoxy nanocomposite. The quality of the dispersion varied with surface modification in the order: COOH > unmodified > PGE, as assessed for SWCNT/epoxy composites using radial breathing modes in Raman spectra and scanning electron microscopy. Local bundling of PGE-modified nanofillers was explained by a polymerization-induced phase-separation process. Dynamic mechanical analysis showed the presence of two relaxation peaks in nanocomposites prepared with unmodified and COOH-modified SWCNT, MWCNT, and CNF, assigned to a partitioning of monomers at the carbon interface. This produced a significant decrease of the glass transition temperature. However, the esterification with PGE led to a single relaxation peak close to the one of the neat epoxy, for the three types of nanofillers without any penalty in the glassy and rubbery elastic moduli. The effect was explained by matrix exclusion from phase-separated domains. POLYM. ENG. SCI., 50:183-190, 2010. (C) 2009 Society of Plastics Engineers