In general, technical vertical electrolysers for the production of chlorine, hydrogen and oxygen are high and have a short cathode-anode distance. Up to the present, few results on mass transfer to gas-evolving electrodes in these industrial cells have been published. The mass transfer experiments were carried out in a divided cell for a vertical hydrogen-evolving platinum electrode in a solution containing 1 M KOH, 0.1 M KCN and 0.008 M AgCN, where the Ag(CN) 2 – complex ion was used as the indicator ion. The platinum electrode was divided into 20 segments, each with a height of 24 mm and a width of 20 mm. Subsequent segments were separated by a 1-mm-thick Perspex layer. The height of the platinum electrode was 0.50 m. It has been found that the Ag/Ag(CN) 2 – redox couple in an alkaline cyanide solution is very useful in determining mass transfer to a hydrogen-evolving electrode. When no hydrogen bubbles are formed at the working electrode, the mass transfer coefficient decreases at a decreasing rate as the distance from the leading edge of the working electrode increases. For a hydrogen-evolving electrode, however, it has been found that the mass transfer coefficient to the topmost 0.40 m of the working electrode is practically constant. For the entrance part of the working electrode, about 0.10 m in length, the dependence of the mass transfer coefficient on the distance to the leading edge of the electrode is complicated. The mass transfer coefficient and the mass transfer enhancement factor for the topmost 0.40 m of the hydrogen-evolving working electrode are given by complex correlations as a function of the current density required for hydrogen evolution and of the flow rate of solution. The effect of viscosity increase of the solution, caused by addition of a polymer, has been investigated. It has been found that the mass transfer coefficient decreases with increasing viscosity of solution in both cases, namely with and without gas bubble evolution, and that the mass transfer enhancement factor is independent of the viscosity of solution.