During ozonation of drinking water, the fungicide metabolite NN-dimethyIsulfamide (DMS) can be transformed into a highly toxic product, N-nitrosodimethylamine (NDMA). We used quantum chemical computations and stopped-flow experiments to evaluate a chemical mechanism proposed previously to describe this transformation. Stopped-flow experiments indicate a pK(a) = 10.4 for DMS. Experiments show that hypobromous acid (HOBr), generated by ozone oxidation of naturally occurring bromide, brominates the deprotonated DMS- anion with a near-diffusion controlled rate constant (7.1 +/- 0.6 X 10(8) M-1 s(-1)), forming Br-DMS- anion. According to quantum chemical calculations, Br-DMS has a pK(a) similar to 9.0 and thus temains partially deprotonated at neutral pH. The anionic Br-DMS- bromamine can react with ozone with a high rate constant (105 2.5 M-1 s(-1)), forming the reaction intermediate (BrNO)(SO2)N(CH3)(2)(-). This intermediate resembles a loosely bound complex between an electrophilic nitrosyl bromide (BrNO) molecule and an electron-rich dimethylaminosulfinate ((SO2)N(CH3)(2)(-)) fragment, based on inspection of computed natural charges and geometric parameters. This fragile complex undergoes immediate (10(10 +/- 2.5) s(-1)) reaction by two branches: an exothermic channel that produces NDMA, and an entropy-driven channel giving non-NDMA products. Computational results bring new insights into the electronic nature, chemical equilibria, and kinetics of the elementary reactions of this pathway, enabled by computed energies of structures that are not possible to access experimentally.