3,5-Dichloro-4H-1,2,6-thiadiazin-4-one 1 condenses rapidly at room temperature with 1,2-diaminobenzene, 2-aminothiophenol and sodium 2-aminophenoxide to give, respectively, the purple thiadiazinoquinoxaline 4a, red thiadiazinobenzothiazine 4b and orange thiadiazinobenzoxazine 4c in almost quantitative yield. The 10H tautomer 4a is spectroscopically almost identical with the purple 10-methyl derivative 11 formed by condensation of 1 with N-methyl-1,2-diaminobenzene and by the methylation of 4a. The chlorine substituent in these tricyclic thiadiazino compounds is readily displaced by nucleophiles, to give the 4-ethoxy 13 and 4-piperidino derivative 21. The dicyanomethylene compound 2, analogous to 1, reacts in the same way as 1 with 2-aminothiophenol to give 4b (87%), but its reaction with 1,2-diaminobenzene is more complex since, in addition to the analogous formation of 4a, there is now the possibility of another cyclisation in the intermediate 16 which leads to the tetracyclic compound 14 and its substitution product 15. This difference in reaction pathway between 1 and 2 is further illustrated by their condensation with 1,8-diaminonaphthalene: 1 is converted into 19 by simple displacement of chlorine, whilst 2 gives, almost quantitatively, the product 20 of cyclisation onto the neighbouring cyano group.