Evidence for the mechanism of thiophene formation from the photolysis of enethiol esters was obtained by studying p-monosubstituted vinylthiol acetates. Styrylthiol acetate gave exclusively 3, 4-diphenylthiophene while P-tert-butylvinylthiol acetate yielded P-tert- butylvinyl disulfide and no thiophenes. These results suggest that the photolysis involves homolytic cleavage of the S-acyl bond followed by dimerization of the enethiyl radical at ...