Under the initiation of benzoyl peroxide the free radical addition of thiophenol to phenylpropiolic acid and its ethyl ester yielded α-phenylthiocinnamic acid (ester), the direction of addition being opposite to that observed with ethoxide-catalysed addition When the reaction with ethyl propiolate and ethyl but-2-ynoate was conducted under the initiation of benzoyl peroxide, the phenylthiyl radical was added to the β-carbon atom.