A novel ring contraction for the conversion of 2, 7-disubstituted 10H-dipyridazino [4, 5-b : 4', 5'-e] [1, 4] thiazine-1, 6-(2H, 7H)-diones (4'a-c) into the corresponding 2, 6-disubstituted 9H-dipyridazino [4, 5-b : 4', 5'-d] pyrrole-1, 5 (2H, 6H)-diones (7'a-c), through a base-induced extrusion of sulfur, is described. Heating of 4'a-c in dimethylformamide in the presence of potassium carbonate smoothly afforded 7'a-c, respectively, in good yields, although neither 2, 8-dimethyl-10H-dipyridazino [4, 5-b : 4', 5'-e] [1, 4] thiazine-1, 9 (2H, 8H)-dione (5'a), nor 3, 7-dimethyl-10H-dipyridazino [4, 5-b : 4', 5'-e] [1, 4] thiazine-4, 6 (3H, 7H)-dione (6'a) was susceptible to similar reaction conditions. The mechanism of the ring contraction, which may involve an intermediate (anion, C^-) containing a thiirane ring, is also discussed.