A mathematical model to represent porous electrodes assuming that the electrochemical reaction has a Tafel kinetic control is proposed. Expressions are developed to analyse the performance of the electrode and the effect of the geometric parameters on the effectiveness is discussed. The theoretical results are compared with experimental data of the oxygen evolution on thick hydrous nickel oxide electrodes under the condition that the resistivity of the pore material is greater than that of the electrolyte solution, rho(m) much greater than rho(s) = 0.