Phosphonium ylides Ph3PC(H)COR (1(H): R = H; 1(Me): Me; 1(Ph): Ph; 1(p-C6H4OMe): p-C6H4OMe; 1(p-C6H4Me): p-C6H4Me; 1(p-C6H4NO2): p-C6H4NO2; 1(OMe): OMe) were prepared by the reaction of triphenylphosphine with the corresponding alkyl halides followed by treatment with a base and applied as novel thermally latent catalysts in the polyaddition of bisphenol A diglycidyl ether with bisphenol A-a model system of epoxy-novolac resins. The phosphonium ylides (1) served as non-salt-type thermally latent catalysts. The relationship between the structure and activity was discussed on the basis of spectroscopic analysis and molecular orbital (MO) calculations. The NMR and IR studies on 1 confirmed that the catalytic activity increased with the electron-accepting character of the acyl group. The MO calculations suggested that a catalyst closer to an ylide structure would be more active, compared with an ylene structure. The catalytic mechanism was also discussed.