Gas-phase thermolysis of thieno [3, 2-e][1, 2, 4] triazines gave benzonitrile, isothiazole, pyrimidine,[1] benzothieno [2, 3-d] pyrimidine and thieno [3, 2-d] thiazole derivatives. A mechanism of these pyrolytic transformation was proposed. Two new and efficient syntheses of the starting thieno [3, 2-e][1, 2, 4] triazines were reported.