Foaming of high-performance material polyphenylsulfone (PPSU) was investigated using supercritical CO₂ or n-butane as the physical blowing agent. The expansion ratio, which ranges from 1.10–16 was obtained under different conditions. Using n-butane is favorable in achieving a high expansion ratio due to its higher solubility and lower diffusivity comparing to CO₂. Temperature is the most important parameter determining the expansion ratio. In batch foaming, the gas saturation time is less sensitive to the expansion ratio. Optimum saturation pressure is essential as the final expansion ratio is very sensitive to the saturation pressure. Similar to saturation pressure, an optimum pressure drop rate is also needed to induce enough thermal instability but not too much to cause cell coalescence and higher open-cell content. Another important parameter is cell growth time right after pressure drop. Due to the low diffusivity of n-butane, the cells grow much slower but larger comparing to CO₂ as PBA.