The electrolytic manufacture of sodium hydrosulfite is affected by feedstock composition, electrode morphology, and electrode material. Here we show that porous graphite cathodes can provide the highest one-pass conversion of sodium bisulfite to hydrosulfite without contamination by sodium thiosulfate. For example, at current densities up to 0.90 kA/m(2), thiosulfate-frec hydrosulfite can be produced at 66% current efficiency. At higher current densities, both hydrogen and thiosulfate are produced. In acetate-buffered bisulfite, these rise to 1.35 kA/m(2) at nearly 100% current efficiency. In unbuffered bisulfite feedstocks, variations in product quality depend solely on cathode composition. That is, the electrode kinetics for each of the reductions are a strong function of the electrode material. However, we show that the rate of bisulfite consumption is controlled by diffusional mass transfer in the porous electrode and can be predicted through use of a simple packed-bed, plug-flow reactor model.