The reaction of benzenethiol with ethylbenzoylacetate in polyphosphoric acid (PPA) at low temperature gave mainly ethyl 3-phenyl-3,3-bis(phenylthio)propionate 8 as an intermediate in the synthesis of thioflavones, while that from t-butylthiobenzene under similar conditions gave ethyl 3-(phenylthio)cinnamate 7 as a major product and many by-products. The reaction constant for intramolecular cyclic