Novel phosphonic acid esters, O,O-di-1-phenylethyl phenylphosphonate (1), O,O-di-tert-butyl phenylphosphonate (2), and O,O-dicyclohexyl phenylphosphonate (3), were synthesized from phenylphosphonic dichloride and the corresponding alcohols. The phenylphosphonic eaters 1-3 decomposed into phenylphosphonic acid and the corresponding olefins by heating at 150-170 degrees C. Their initiator activities were examined in the cationic polymerization of glycidyl phenyl ether (GPE). They converted GPE only 4% even at 190 degrees C in the absence of ZnCl2. In the presence of ZnCl2 along with 1-3, GPE did not convert below 90 degrees C in the case of 1 and 2, and below 140 degrees C in the case of 3, but it rapidly converted to afford polyGPE with M-n of 2000-7000 above those temperatures. It was found that 1-3 served as thermally latent initiators in the polymerization of GPE in the presence of ZnCl2.