Reaction of N'-phenylthioformohydrazide with carbonyl compounds, such as aliphatic, aromatic and heterocyclic aldehydes and a ketone, in the presence of trimethylsilyl chloride proceeded easily to afford the corresponding 2, 2'-substituted-2, 3-dihydro-3-phenyl-1, 3, 4-thiadiazoles in good yields under mild conditions.