The synthesis and crystal structures of three mercapto functionalised 1, 3, 4-thiadiazoles and the crystal structure of 2-mercapto-5-methyl-1, 3, 4-thiadiazole are described. In the solid state, 2-mercapto-5-methyl-1, 3, 4-thiadiazole 1 forms a thioamide tautomer as shown by FTIR and Raman spectroscopy as well as X-ray crystallography and as theoretically predicted. The molecules are connected to form chains via N–H⋯ S hydrogen bonds with ...