N,N-Dimethylsulfamide (DMS), a newly identified, ubiquitous degradation product of the fungicide tolylfluanide, has been shown to be a N-nitrosodimethylamine (NDMA) precursor during zonation. In this study, batch ozonation experiments in ultrapure buffered water, surface water, and tap water were performed to determine the kinetics and elucidate the mechanism of NDMA formation from DMS. It was found that at circumneutral pH, DMS reacts slowly with ozone (k approximate to 20 M-1 s(-1)) and moderately with hydroxyl radicals (k = 1.5 x 10(9) M(-1)s(-1)). The reaction of DMS with these oxidants does not lead to NDMA. NDMA was only formed if bromide was present during zonation of DMS-containing waters. Bromide is oxidized to hypobromous acid (HOBr) by ozone which then reacts with the primary amine of DMS to form a Br-DMS species. The rate limiting step of the formation of Br-DMS is the formation of HOBr. The reaction to form Br-DMS has an apparent second order rate constant at pH 8 of >3 x 10(4) M(-1)s(-1). The Br-DMS is transformed by ozone to NDMA and nitrate (k >= 5000 M-1 s(-1)), with yields of 54% and 39%, respectively, based on the primary amine nitrogen of DMS. These reactions release bromide, making bromide a catalyst NDMA is also formed during zonation of DMS in the presence of hypochlorous acid (20-30% yield). The last step of NDMA formation is an intramolecular rearrangement with sulfur dioxide extrusion. On the basis of the mechanistic and kinetic information, it was possible to model NDMA formation in DMS-containing Lake Zurich water.