To obtain the subject PPE/PS resin beads capable of giving expanded resin molded forms with such high heat resistance and flame retardancy as to stand the burning test specified in the JIS A9511 Standard, and of also giving expanded resin molded forms with high expansion ratio, thereby having physical properties necessary as expanded molded forms such as low density and favorable surface character.

The resin beads are obtained by impregnating a basal resin comprising 60-90 pts.wt. of a styrene-based resin and 40-10 pts.wt. of a polyphenylene ether-based resin with 3-15 wt.%, based on the basal resin, of a volatile foaming agent and also incorporating the basal resin with 1-20 wt.%, based on the basal resin, of a brominated aromatic triazine compound, in particular 2,4,6-(tris-2,4,6-tribromophenoxy)-1,3,5-triazine.