Processing of a high glass transition (T-g) polymer such as polyethersulfone (PES; T-g=225 degrees C) poses a challenge as it requires high processing temperatures or sometimes toxic solvents. In this work, we report of a facile process using superheated water (shH(2)O) and supercritical carbon dioxide (scCO(2)) co-media. PES solids were foamed in the scCO(2)/shH(2)O co-media or scCO(2) alone in a batch process at different temperatures. The scCO(2)/shH(2)O produced a synergistic effect and achieved PES foams even at processing temperatures as low as 85 degrees C below the nominal T-g; whereas, scCO(2) alone required higher processing temperatures. Moreover, the scCO(2)/shH(2)O co-media produced highly porous PES foams that were at least 23% higher in porosity than what was obtained using scCO(2) alone. In addition, the scCO(2)/shH(2)O produced open cell foams at some processing conditions; whereas, scCO(2) produced closed cell morphologies. Since both CO2 and H2O are innocuous, this approach has potential for use in the preparation of ultrafiltration membranes, which currently require the use of toxic solvents for their fabrication by way of the phase inversion process. Moreover, the use of scCO(2)/shH(2)O is a cost-effective approach for the processing of high T-g polymers at significantly lower temperatures. POLYM. ENG. SCI., 58:1108-1114, 2018. (c) 2017 Society of Plastics Engineers