The dipotassium salt of 3,4-dihydroxy-1,2,5-thiadiazole 1,1-dioxide (4a) was synthesized in high yield from sulfamide and diethyl oxalate. The free acid (7) was prepared from 4a, from the disilver salt (4c), or from 3,4-dichloro-l,2,5-thiadiazole 1,1-dioxide ( 13), the latter being synthesized from 4a and phosphorus pentachloride. The reactive 13 was converted in methanol to the dimethoxy derivative (12). Either 12 or 13 reacted with ammonia to form the 3,4-diamino derivative ( 14) and with methylamine, dimethylamine, and ethylenediamine, respectively, to produce the 3,4-bis(methylamino) (15), the 3,4-bis(dimethylamino) ( 16), and the 3,4-piperazino (23) derivatives. One mole of morpholine with 12 yielded 3-morpholino-4-methoxy-l,2,5-thiadiazole 1,1-dioxide ( 17), which could be rearranged smoothly by heating to 2-methyl-3-oxo-4-morpholino-l,2,5-thiadiazoline 1,1-dioxide (18). Two moles of piperidine with 12 gave 3-oxo-4-piperidino-l,2,5-thiadiazoline 1,1-dioxide ( 19) and IV-methylpiperidine. Dimethoxy derivative 12 rearranges thermally, first to 2-methyl-3-oxo-4-methoxy-l,2,5-thiadiazoline 1,1dioxide (10) and then to 2,5-dimethyl-3,4-dioxo-l,2,5-thiadiazolidine 1,1-dioxide (11). o-Phenylenediamine with 12 in DMF gave the tricyclic l,3-dihydro[l,2,5]thiadiazolo [3,[4][5][6]quinoxaline 2,2-dioxide (24). 12 and 14 condensed in the presence of sodium methoxide to form a linear tricyclic quinonoid salt (25a). 13 reacted with 2 mol of anthranilic acid to yield a diamine (26) which was dehydrated to a linear pentacyclic bis(quinazolino)-l,2,5-thiadiazole derivative (27).