Substituent effects of phosphonium ylides Ph3PC(H)COR (1(R)) were kinetically studied in the reaction of glycidyl phenyl ether (GPE) with 2,6-dimethylphenol in the presence of the phosphonium ylide catalysts, (formylmethylene)triphenylphosphorane (1(H)), (acetylmethylene)triphenylphosphorane (1(Me)), (benzoylmethylene)triphenylphosphorane (1(Ph)), and (pivaloylmethylene)triphenylphosphorane (I-tBu) for molecular design of excellent latent catalysts. Both the activation energy (E-a) and frequency factor (A) of the reaction increased with the bulkiness of the acyl group of 1. Further, the catalytic activity was examined in the polyaddition of bisphenol A diglycidyl ether with bisphenol A in the presence of phosphonium ylide catalysts. The catalytic activity decreased with the bulkiness of the acyl group of 1, but the latency, i.e., the starting temperature, of the epoxide conversion was little affected. On the other hand, the introduction of acetyl and cyano groups into the catalysts as the electron-withdrawing moieties shifted the temperature-conversion curves to higher temperature region compared with the case of 1. The thermal latency of the phosphonium ylides was discussed from the viewpoint of the catalytic mechanism with the kinetic data.