The aim of this work was to investigate the foaming process of high-performance thermoplastic polymers such as polyethersulfone (PES), polyphenylsulfone (PPSU), polyetherimide (PEI), and poly(ethylene-2,6-naphthalate) (PEN) expanded by using supercritical carbon dioxide as a blowing agent. All polymers were characterized by differential scanning calorimetry (DSC) and rheological analysis to roughly identify the foaming conditions. Batch and solid-state foaming methods were employed. In the first case, cell nucleation was promoted by inducing a fast pressure drop rate in a pressurized vessel. In the solid-state process, foaming was promoted by increasing the temperature of gas-saturated samples in an oil bath. The effects of foaming methods and process parameters on cellular morphology were analyzed. All polymers were successfully foamed by using the solid-state technique, showing relative densities ranging from 0.13 to 0.44 for PEN and from 0.27 to 0.57 for PES, PPSU, and PET. The morphology was microcellular in all cases, and PES exhibited nanocellular cells after some processing conditions. The batch-foaming process was less effective to prepare foams than the solid-state one. In fact, higher relative densities and reduced temperature windows for foaming were evidenced for amorphous polymers, whereas PEN crystallized during the heating step and foams with poor morphology and high relative densities were obtained. (C) 2011 Wiley Periodicals, Inc. Adv Polym Techn 30: 234-243, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/adv.20219