An unusual ring-expansion reaction of 4-amino-1,1-dioxo-[1,2,3,5]-thiatriazoles 4 has been identified that produces the relatively rare 5-amino-1,1-dioxo-[1,2,4,6]-thiatriazines 6 and 9. Initial alkylation of the thiatriazole 4 with α-halo-esters at N-3 produces α-substituted esters which, under basic reaction conditions, undergo opening of the thiatriazole ring and re-closure to a thiatriazine ring. Similar alkylations of 4 with diethyl chloromalonate and ethyl dichloroacetate lead to the loss of SO₂ and the production of triazine 10a and triazole 14, apparently by an initial alkylation at N-5. The reaction of 4 with phenacyl bromides or a phenacyl dibromide forms fully unsaturated 5-amino-1,1-dioxo-[1,2,4,6]-thiatriazines 13.