Cyclization of 2-substituted 5-[(α-bromobenzylidene) hydrazino)]-4-chloro-3 (2H)-pyridazinones (2) with potassium thioacetate, followed by deacetylation, provided new ring system derivatives, 7-substituted 2-phenyl-4H-pyridazino [4, 5-e] [1, 3, 4] thiadiazin-8 (7H)-ones (4). 8-Chloro and 8-amino derivatives (10) of the pyridazinothiadiazine ring were readily derived from 4-acetyl-2-phenyl-4H-pyridazino [4, 5-e] [1, 3, 4] thiadiazin-8 (7H)-one (3d) by chlorination and subsequent amination. Ring contraction of the 8-oxo derivatives (4) to 5-substituted 3-phenyl-1H-pyrazolo [3, 4-d] pyridazin-4 (5H)-ones (6), through extrusion of sulfur under basic or thermal conditions, was observed. A similar reaction occurred in the case of the 8-chloro and 8-amino derivatives (10). Probable mechanisms for these reactions and differences of reactivity between 4 and 10 are discussed.