The foaming processes of nanocomposites based on high-performance thermoplastic polymers, namely, poly(ether sulfone) (PES; amorphous) and poly (ethylene-2,6-naphthalate) (PEN; semicrystalline), reinforced by two nanofiller types (expanded graphite and SiO(2) nanopowder), were investigated. Matrices were prepared by melt blending through extrusion, and a good dispersion of particles was achieved, as confirmed by microscopic and X-ray diffraction analyses. A solid-state foaming technique was used to prepare the foams; the samples were solubilized with carbon dioxide and quickly heated in an oil bath to the selected foaming temperature. The effects of both the type and concentration of the filler and the polymer type (amorphous and semicrystalline) on the cellular morphology were analyzed. Foams prepared from PES-based nanocomposites showed microcellular morphologies and higher numbers of nucleated cells (up to 10(11) cells/cm(3)), but low expansion ratios were achieved compared to PEN-based foams. Both SiO(2) and graphite nanoparticles acted as cell nucleating agents in the PES nanocomposites, but the latter gave better results, increasing the cell number by two orders of magnitude with respect to the neat polymer. This behavior was attributed to either the heterogeneous nucleation of cells or the improved barrier to gas diffusion of the graphite nanoplatelets with respect to SiO(2) nanoparticles. The PEN nanocomposite foams exhibited low foam densities, but fewer cells were nucleated with respect to the PES nanocomposites. The increase in the crystallization rate related to the presence of fillers, in particular when graphite was used, affected the expansion ratio at high foaming temperatures. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 3701-3711, 2011