A series of thieno[3,2-e]-3,4-dihydro-1,2,3-triazin-4-ones (V) and thieno[3,4-e]-3,4-dihydro-1,2,3-triazin-4-ones (VI) were prepared by diazotation of 3-aminothiophene-2-carboxamides (III) and 4-amino-thiophene-3-carboxamides (IV), respectively, both obtained by cyclization of 2-cyanoethylene-1,1-dithiolates with α-halogenocarbonyl compounds. Characteristic differences between the two systems were found in the position of the long-wavelength UV absorption maximum and the acidity of the triazine ring proton. The latter property was reflected in the ¹H NMR signal from this proton.