Parallel plate electrolysers are analysed using computational fluid dynamics, CFD, with emphasis on the region close to the surface of the cathode. The numerical solution takes place inside the diffusive layer where chemical reactions occur. Numerical predictions in this region are proposed to obtain detailed calculations of electroactive species concentration (Cu-2), C, which is affected by the hydrodynamic behaviour of the electrolyte. The concentration C plays a main role since it is related to the mass transport coefficient, km, in order to define the efficiency of the reactor. This approach allows the designer of electrolysers avoiding the use of correlations where the Reynolds (Re), Sherwood (Sh) and Schmidt (Sc) numbers based on average velocity are used to calculate the value of C and km. Therefore a fully developed flow in the reactor is not a condition, since the value of C and km is calculated with the actual velocity of the electrolyte in the boundary layer, at a non-dimensional distance to the wall as small as y(+) approximate to 5. The results are compared against data reported in the literature showing good agreement. Therefore, a solution of the hydrodynamics and the electrochemistry, link together in the near wall region, is obtained by refinement of the computational grid. (C) 2015 Elsevier B.V. All rights reserved.