The reactions of aminoiminomethanesulfonic acid, phenylaminoiminomethanesulfonic acid, and N,N'-diphenylaminoiminomethanesulfonic acid in aqueous media at pH 7.4, 10, and 13-14 were investigated. At neutral pH hydrolysis to the corresponding urea was the major pathway for all three compounds. At higher pH phenylaminoiminomethanesulfonic acid reacted to give phenylcyanamide in nearly quantitative yield, while N,N'-diphenylaminoiminomethanesulfonic acid gave diphenylcarbodiimide which reacted further to give N,N'-diphenylurea. At pH 10 aminoiminomethanesulfonic acid reacted with itself, eventually giving N-cyanoguanidine, while at pH 13-14, elimination to cyanamide predominated. The reactions of glycine with the aminoiminomethanesulfonic acids gave guanylated acetic acids as products. The rates of these nucleophilic substitutions of the sulfonic acid group of the aminoiminomethanesulfonic acids by the amino group of glycine decreased in the order aminoiminomethanesulfonic acid greater than phenylaminoiminomethanesulfonic acid greater than (2-methylphenyl)aminoiminomethanesulfonic acid greater than (2,6-dimethylphenyl)aminoiminomethanesulfonic acid. Higher relative rates of substitution of the aminoiminomethanesulfonic acids appear to be related to higher relative toxicities for the corresponding thioureas.