Flow dispersion, pressure drop, and averaged mass transport measurements have been made to characterize the reaction environment in an industrial scale electrochemical reactor. The 10 cell filter-press stack was operated with a relatively low mean linear velocity in the range 0.6 to 6.2 cm s–1. Flow dispersion was studied by a perturbation–response technique by electrolyte conductivity measurements at the reactor outlet. Mass transport coefficients were evaluated from the first order reaction decay of dissolved bromine (Br3–) which was anodically generated from 1 mol dm–3 NaBr (aq). Each cell consisted of two 0.72 m2 projected area electrodes separated by a cationic membrane, and each electrolyte compartment contained a high-density polyolefin turbulence promoter. The electrodes consisted of a carbon/polyethylene core with a layer of an activated carbon–poly(vinylidene difluoride) composite on each side. Comparison is made with the mass transport characteristics of a similar system that contains five bipolar cells.