Three synthetic routes to salts of 5-amino-5-hydroxy-2,4,6(1H,3H,5H)-pyrimidinetrione (10) are described. The key reactions involved acid-catalyzed cleavage of 5-amino-5-ureido-2,4,6(1H,3H,5H)-pyrimidinetrione (7), conversion of uramil (8) to dehydrouramil (9) and subsequent hydration, and the condensation of alloxan (5) with ammonium salts. The carbinol ammonium salt structure 10a was unambiguously established by X-ray crystallography. New alloxan-like compounds 7, 9, and 10 were evaluated for diabetogenic activity in rats. Compound 7 was inactive, whereas compounds 9 and 10 showed the highest activity comparable to that of streptozotocin (12).