Composite polystyrene foam with a honeycomb-like barrier structure was processed from an expandable aqueous suspension. Optical observations confirmed the formation of such a unique structure that encapsulates each expanded polystyrene microsphere in the resulting foam. The suspension viscosity was found to highly influence the foam morphology. Results from mechanical tests showed that the existence of the barrier structure can considerably improve the mechanical performance of the composite foam. Fire retardation tests demonstrated that the barrier structure can effectively stop the fire path into the foam, suppress toxic smoke generation, and maintain structural integrity, leading to improved fire resistance in comparison with the neat polystyrene foam. POLYM. ENG. SCI., 55:1494-1503, 2015. (c) 2015 Society of Plastics Engineers