Certain new poly(silyl ether)s and a poly(germyl ether) were synthesized by the polyaddition of bisepoxides with dimethyldiphenoxysilane (MPS) and dimethyldiphenoxygermane (MPG). The polyaddition of Bisphenol A diglycidyl ether (BPGE) with MPS proceeded very smoothly and regioselectively using quaternary onium salts such as tetrabutylammonium chloride (TBAC) and tetrabutylphosphonium chloride (TBPC) as catalysts at 100 degrees C to give the corresponding poly(silyl ther)s having high molecular weights, although the reaction did not occur without a catalyst under the same reaction conditions. The polyaddition also proceeded using either triphenylphosphine (TPP), triethylamine (TEA), or a complex of 18-crown-6 with potassium chloride as catalysts to give the poly(silyl ether)s, but with low molecular weights. The polyaddition of BPGE with MPG produced the corresponding poly(germyl ether) with good yield using TBAC as a catalyst; however, the molecular weight of the isolated poly(germyl ether) was lower than that of the obtained poly(silyl ether) under the same conditions.