A system of four equations is developed to simulate gas evolution reaction within flow-through porous electrodes. The equations include the mass transfer resistance and gas bubble formation at definite kinetic and ohmic parameters. Two cases are introduced: the first is when the bubble effects are separated from the mass transfer effects; the second, is the combined bubble and mass transfer effects. These interlinked effects are discussed by examining the overall performance of the electrode and the distributions of the gas void fractions, potential and reaction currents. When the bubble effects are included in the simulation, the features of the polarization curves as well as the current distributions are significantly influenced. The gas bubbles decrease the effective conductivity of the gas-electrolyte dispersion filling the pore space resulting in a more non-uniform distribution of the potential and current.