Currently, most commercially available polymeric foams are manufactured with commodity plastics, such as polyvinyl chloride, polystyrene, polypropylene, and polyethylene, which have limited applications largely due to their low service temperatures. Subsequently, foam processing of advanced engineering polymers with outstanding thermal stability and mechanical properties has received a great interest for the potential of broadening the spectrum of polymeric foam usage. In this aspect, the present study examines the foaming ability of Polysulfone (PSU), and Polyethersulfone (PES), which are aromatic thermoplastics exhibiting superior thermal capabilities. Experiments are performed via the one-step batch foaming process with two different physical blowing agents: supercritical CO2 and argon. In the case of CO2, a typical “mountain” shaped expansion ratio with regard to foaming temperature is observed. Throughout the temperature range examined, both PSU and PES develop uniform microcellular structures with high nucleation densities up to 1010 cells/cm3. In the case of argon, expansion ratios at varying foaming temperatures show a distinctive trend of a two peak “mountain” shape. The study also presents a unique phenomenon observed from cellular structures of foamed PSU and PES: development of nanostructures on cell walls. Experiments with CO2 and argon both produce protruding nano-fibrillar network and rippled topography on the cell walls of PSU and PES foams, respectively. The work discusses the mechanism behind the formation of nanostructures which may attribute to the high foamability verified from CO2 experiments and the peculiar two peak expansion ratio behavior observed from argon.