Two-step electrode mechanisms that are associated with chemical equilibria are considered theoretically under conditions of square-wave voltammetry. The mechanisms designated as "surface EE", "surface ECE", "surface EECrev" and "surface EECatalytic" mechanisms are often encountered in experimental chemistry of many drugs and redox enzymes containing ions of transient metals as co-factors, or quinone-moieties, as well. We present large set of representative results that reveal specific features met at all considered mechanisms. We also propose set of simple methodologies to get access to kinetic and thermodynamic parameters relevant to electron transfer step, and to associated chemical reactions, as well. The work is of outmost importance to understand many aspects of enzyme-substrate interactions, but also the nature of drug-drug interactions of lipophilic and hydrophilic drugs
