If both a reactant and product of an electrode reaction are present in the bulk of solution, the square-wave voltammogram can be recorded by starting at the equilibrium potential. If the reaction is reversible, the redox state of investigated substance can be estimated from the ratio of peak currents of the forward and backward components. The degree of reversibility of the reaction can be assessed from the current at the starting potential. Also, the dependence of the logarithm of forward component of irreversible response on the electrode potential can be used for the calculation of transfer coefficient. (C) 2013 Elsevier B.V. All rights reserved.