In the production of chlorine and hydrogen from hydrochloric acid by electrolysis in an electrolysis cell comprising a plurality of vertically arranged bipolar electrodes, a diaphragm arranged between each two electrodes to divide the electrolysis chambers formed between them into an anolyte chamber and a catholyte chamber, and outlet and inlet devices for the electrolyte, the improvement which comprises electrolyzing the hydrochloric acid in at least two successive stages, and degassing the hydrochloric acid. Advantageously the hydrochloric acid moves from bottom to top, first through an upper stage and then through a lower stage. As a result less electrode surface is needed, a higher current density and/or voltage is possible so existing apparatus can be modified to connect more bipolar electrodes in series.