Reaction of 1,3-di-tert-butylaziridinone (1a) and similar aziridinones with thiosemicarbazide affords, as one of the products, a compound devoid of sulfur, viz., a substituted N-aminoimidazolidinone (3a) by selective cleavage of the acyl-nitrogen bond. Compound 3a undergoes a novel, acid-catalyzed rearrangement to a 3-imino-hexahydro-1,2,4-triazin-6-one (7a), which can also be obtained by treatment of 1a with hydrazine followed by BrCN, involving again selective cleavage of the acyl-nitrogen bond.