In this study, a benign process was used to successfully produce low density foam from poly(arylene ether sulfone) (PAES). Both carbon dioxide (CO2) and water as well as nitrogen and water were used as physical blowing agents in a one-step batch process. A large amount of blowing agents (up to 7.5%) was able to diffuse into the PAES resin in a 2-h saturation time. Utilizing water and CO2 as the blowing agents yielded foam with better properties than nitrogen and water because both the water and CO2 are plasticizers for the PAES resin. PAES foam produced from CO2 and water had a large reduction in foam density (similar to 80%) and a cell size of similar to 50 mu m, while maintaining a primarily closed cell structure. The small cell size and closed cell structure enhanced the mechanical properties of the foam when compared with the PAES foam produced from nitrogen and water. The tensile, compressive, and notched izod impact properties of the PAES foams were examined, and the compressive properties were compared to commercially available structural foams. With reduced compression strength of 39 MPa and reduced compression modulus of 913 MPa, the PAES foam is comparable to polyetherimide and poly(vinylchloride) structural foams. POLYM. ENG. SCI., 49:44-51, 2009. (C) 2008 Society of Plastics Engineers