It is known that electrolytic processes involving the production of gas may have a low yield energetic due to the influence of the bubbles formed on specific electrode geometry. These bubbles adhering the electrode surface increases its area of coverage may affect the average mass transfer coefficient and hence the cell potential and the yield process. In the present work, the combined average mass transfer coefficient was determined for the hydrogen production from the electroreduction of K3Fe(CN)(6) in alkaline solution under diffusional control. Like a reactor to produce electrolytic chlorine-soda, the electrolytic cell was formed by anodic and cathode compartments separated by an asbestos diaphragm. The effects of electrolyte percolation velocity and cathode geometry (perforated plate and metal mesh) were studied. The results showed that for the mesh geometry, the combined average mass transfer coefficient (k(d)*) over bar increased with the average electrolyte percolation velocity through the diaphragm. A contradictory trend was observed for the perforated plate. For average percolation velocities below 3.5 x 10(-5) m s(-1). the perforated plate showed higher values of (k(d)*) over bar than the mesh geometry. The constants a and b of the well-known dimensionless equation Sh = aRe(b)Sc(1/3) were presented for the cases studied. Published by Elsevier B.V.