Styrene-divinylbenzene copolymer containing N-bromosulphonamide groups as sodium salt, [P]-SO2NBrNa, can be viewed as a high molecular weight redox polymer mimicking its low molecular weight analogue: Bromamine-T. The stability of this copolymer in acidic media has not been studied in detail to date. This paper reports on studies of the reaction between the copolymer and aqueous FeSO4 solution conducted in a series of batch and column experiments. It was found that pH <3.0 results in transition of active bromine from -SO2NBr- groups to the solution, followed by oxidation of Fe2+ ions to Fe3+ ions by elemental bromine. In reaction media of pH >3.0, active bromine remains bound to the functional groups, facilitating application of this copolymer as an effective oxidant in column processes running in weakly acidic reaction media. Results of the present work provide evidence that -SO2NBr- functional groups of the copolymer can stably exist in acidic form which is in contrast to the behaviour of the low molecular weight analogue (N-bromo-p-toluenesulphonic acid does not occur as a pure chemical compound).