The effect of radial diffusion on the polarization of porous flow-through electrodes has been investigated with the aid of a mathematical model. The proposed model takes into consideration the rates of mass transfer in the axial direction by convection and in the radial direction by diffusion as well as charge transport in the pore electrolyte and electron transfer kinetics at the electrode-electrolyte interface. Normalization of the variables gave rise to dimensionless groups pertinent to the kinetic, ohmic and radial diffusion effects. These are respectively,I the reversibility index, Δ the parameter of ohmic effect andψ the parameter of radial diffusion. The latter (ψ=2φ/Sh) is the ratio of two other dimensionless groups. With this formulation, larger values ofψ correspond to more predominant control of the electrode behaviour by radial diffusion. The same is also true for the parameter of ohmic effect Δ. Solutions have been obtained for two limiting cases: negligible and signifcant potential drop in the pore electrolyte. In both cases, equations have been derived which give the quantitative (highly non-linear) effect ofψ on the current-polarization relations. In the case of a significant ohmic potential drop in the pore electrolyte, it was found that the controlling parameter is the product Δψ The two variables seem to give a synergistic effect since, at large Δ values a certain change inψ has a more pronounced effect on the polarization than the corresponding change at lower Δ values. Qualitative and quantitative tests of some aspects of the model are reported using the electrochemical reduction of copper ions from acid copper sulphate solutions at a packed bed of copper particles. Satisfactory agreement was obtained.