Abstract: Regeneration of spent hydrochloric acid after use for pickling iron, which herefore contains iron chlorides, by electrolysis in the cathodic and intermediate compartments of an electrodialysis cell. The installation includes electric circuitry for connecting up the anode as well as input conduits for the spent acid but otherwise consists essentially of an electrodialysis cell with an uneven number of compartments wherein each cathodic compartment is separated from the adjacent intermediate compartment by an anion-selective membrane while a cation-selective membrane separates each intermediate compartment from the anodic compartment, the latter being filled with an anolyte whose total Faraday yield for water oxidation is close to 1. Electrolysis is conducted with current densities at the cathode of 0.1 - 20 A/dm.sup.2, at the anode corresponding to the water oxidation reaction and at the membranes such that their perm-selectivity is close to 1.