Abstract Some derivatives of 1-carbamoylhydantoins undergo cleavage at the N1–C(O) bond on heating in solution. 1-Carbamoylhydantoins in aqueous and alcoholic ammonia rearrange via opening of the imidazole ring, followed by recyclization. Prolonged treatment of 1-carbamoylhydantoins with aqueous ammonia can lead to elimination of the C2=O carbonyl group.