4-(2-Aminophenylthio)-3, 6-dichloropyridazine (I) on treating with dil. Hydrochloric acid or acetic acid gave 3-chloro-5H-benzo[b]pyridazino[4, 3-e][1, 4]thiazine (3, 4-diazapheno-thiazine type) through rearrangement and cyclization. 3-Chloro-10H-benzo[b]pyridazino-[3, 4-e][1, 4]thiazine (1, 2-diazaphenothiazine type) was obtained by either heating of I at 150°or treating of I with conc. hydrochloric acid through direct cyclization. The mechanism of the aforesaid reactions was discussed using a molecular orbital method. Several 1, 2-and 3, 4-diazaphenothiazine derivatives were synthesized.