The kinetics of the formation and decomposition of hydroxymethanesulfonate (HMSA) have been examined at pH 4 and 5. The second-order rate constant for the formation of HMSA at 25¿C was measured to be 1.94¿0.11 and 12.60¿1.04 mol-1 s-1 at pH values of 4 and 5, respectively. The rate constant of HMSA decomposition to form formaldehyde (HCHO) and aqueous sulfur dioxide S(IV) at pH 4 and 5 was 4.8¿0.410-7 and 3.5¿0.2¿10-6 s-1, respectively. In addition, no reaction between hydrogen peroxide (H2O2) and HMSA was observed when a tenfold molar excess of H2O2 was present at pH values of 4 and 5.