The reaction between acidic bromate and aminomethanesulfonic acid (AMSA) has been studied in strongly acidic media. The reaction is very slow, and typically goes to completion after about 600 min at pH 0.0 and a [BrO3-]/[H2NCH2SO3H] ratio of 100. The reaction was followed by observing the absorbance of aqueous bromine at lambda = 390 nm and the formation of an intermediate bromamine species at lambda = 330 nm. The stoichiometry is 3H(2)NCH(2)SO(3)H + 2BrO(3)(-) + 4Br(-) + 6H(+) --> 3Br(2)NCH(2)SO(3)H + 6H(2)O. There is no cleavage of the C-S bond. Electrophilic substitution, instead, takes place at the nitrogen center to give bromamine derivatives as products. Dibromamine is the favored product over the monobromamine in high acid environments. Bromine, which is formed during the reaction, slowly reacts with AMSA to form a mixture of monobromoaminomethanesulfonic acid and dibromoaminomethanesulfonic acid. Initially, pure oxyhalogen chemistry produces bromine which then takes part in the electrophilic substitution reaction. An Ii-step mechanism is proposed. The model derived from the mechanism gives relatively good agreement with experimental data.