2-Dimethylamino-5-phenyl-1, 3, 4-thiadiazin-6-one (), the first representative of this ring system, undergoes thermal fragmentation to yield benzonitrile, dimethylcyanamide, and 2- dimethylamino-5-phenyl-1, 3, 4-thiadiazole; its reaction with 1-diethylamino-propyne results in a mixture of adducts, the 1-pyridazin-4-one