A new effective diffuse layer thickness dependent surface roughness model, based on the non-linear Poisson-Boltzmann theory, has been used for the interpretation of impedance data for rough cadmium electrodes. The so-called electrochemical roughness function values at various electrode potentials and surface charge densities have been calculated. It was found that the roughness function values, the inverse value of Parsons-Zobel plot, as well as the fitting coefficient in the Valette-Hamelin approach, obtained from the electrochemical impedance data, art: effective (seeming) parameters and do not characterise the real geometrical structure of electrode surface. However, they characterise the deviation of real experimental system from ideally smooth (flat) surface. In the region of moderate rational electrode potentials 0 less than or equal to \E\ less than or equal to 0.2 V and surface charge densities (1 less than or equal to \a\ less than or equal to 4 muC cm(-2)) the roughness function rises with the decrease of the effective thickness of diffuse layer. At higher electrode potentials and surface charge densities the surface roughness function levels of to the constant value, i.e. to the so-called geometrical surface roughness value. The new effective diffuse layer thickness dependent surface roughness theory needs future corrections taking into account the role of crystallographic inhomogeneity of the geometrically rough electrode surface, i.e. the dependence of the local surface charge density on the crystallographic structure of energetically homogeneous regions exposed on the macro-polycrystalline electrode surface. (C) 2000 Elsevier Science Ltd. All rights reserved.