The cationic polymerization of glycidyl phenyl ether (GPE) was examined with novel phosphonic acid esters, O,O-di-p-methylbenzyl phenylphosphonate (la), O,O-dibenzyl phenylphosphonate (1b), and O,O-di-p-nitrobenzyl phenylphosphonate (Ic), as initiators. GPE did not convert with 1a-c even at 210 degrees C in the absence of ZnCl2. In the presence of ZnCl2 along with phosphonic acid esters la, Ib, and Ic, the polymerization of GPE did not proceed below 90, 130, and 170 degrees C, respectively, but it rapidly proceeded to afford polyGPE with M-n of 2500-5000 above those temperatures. It was found that la, Ib, and Ic seri ed as thermally latent initiators in the polymerization of GPE in the presence of ZnCl2. Electron-donating substituents on the benzyl group of the phosphonic acid esters increased the initiator activity. The initiation species was identified as a benzyl cation by C-13-labeled experiments.