Abstract Aminoiminomethanesulfonic acid, H 2 NC ( NH) SO 3 H, converts primary amines to the corresponding guanidine derivatives at 20 C. This crystalline reagent is readily prepared by the peracetic acid oxidation of fonnamidinesulfim'c acid, H 2 NC ( NH) SO 2 H. The synthesis is illustrated by a representative group of simple monosubstituted guanidines shown in Table I.