Microcellular foams from high-performance poly(phenylene sulfide)/poly(ether ether ketone) (PPS/PEEK) blends were fabricated using environment-friendly supercritical CO2 (scCO(2)) as a physical foaming agent. The gaseous mass-transfer, microcellular foaming and properties of PPS/PEEK blends as functions of multiphase composition and process conditions were systematically studied. It is found that heterogeneous microcellular structures with much larger cell density and smaller cell size were obtained due to the crystal structures, phase interfaces, different viscosity and gas concentration in PPS and PEEK phases. The mechanisms of gas-induced cold crystallization and thermally induced crystallization of the polymer/gas systems have been elaborated in detail. Compared with the solid PPS/PEEK blends, the microcellular ones exhibit higher crystallinity, specific tensile strength and impact strength, and decreased storage modulus and loss factor. The obtained results indicate that microcellular foaming of PPS/PEEK blends is an effective way for preparing lightweight and high-performance polymer foams with tailored morphologies and excellent properties.