A complex electrode mechanism coupled with adsorption of the redox couple and an irreversible homogeneous chemical reaction that regenerates the electroactive reactant is studied both theoretically and experimentally under conditions of square-wave voltammetry. The homogeneous regenerative redox reaction, i.e., the so-called catalytic reaction, takes place as a surface or volume process, depending on whether it includes the dissolved or adsorbed form of the electrode product, respectively. A rigorous theoretical model is presented considering simultaneously all relevant phenomena affecting the voltarnmetric response, such as mass transport, adsorption equilibria and kinetics of two different catalytic reactions, i.e., volume and surface catalytic reactions. The solutions for the surface concentration of the electroactive species are presented in the form of integral equations, thus they are general and valid for any chronoamperometric and voltammetric technique. The model enables systematic study of the role of adsorption in a complex catalytic electrode mechanism. The properties of the voltarnmetric response show remarkable discrepancies between the effect of the volume and surface catalytic reactions, thus enabling their recognition and separate characterization. From an analytical point of view it is demonstrated that the catalytic mechanism of moderate adsorption is superior in comparison with the pure volume and surface catalytic mechanisms. The theoretical predictions are confirmed by the voltarnmetric behaviour of the redox couple azobenzene/hydrazobenzene at the mercury electrode by using a mixture of water + acetonitrile as a solvent. (c) 2005 Elsevier B.V. All rights reserved.