In this work, a microwave expansion process to produce thermoset-matrix syntactic foam containing thermoplastic foam beads was designed and developed. Expandable polystyrene (EPS) microspheres and epoxy resin were chosen as a model material system. This process is featured with a capability to effectively expand EPS microspheres in syntactic foam with high EPS loading. The resin viscosity and specific microwave energy are found to be the two primary control parameters determining the process window. Mechanical characterization showed that the specific flexural strength and modulus of the syntactic foam are similar to those of the neat epoxy. By comparison, the flexural moduli over density squared or cubed of the foam are much higher, especially at high EPS loadings, than those of the neat resin. The foamed EPS microspheres can also effectively toughen the syntactic foam, preventing propagation of cracks. Furthermore, the microwave expansion process was found to be capable of molding syntactic foam parts of relatively sophisticated geometry with smooth surfaces.(C) 2014 Society of Plastics Engineers