Equations describing the development of concentrations in an EE two-electron-transfer redox system with comproportionation/disproportionation during electrolysis are solved. The change of the open circuit potential of an indicator electrode under such conditions is analysed as a function of the charge Q transferred and the disproportionation equilibrium constant K. The analysis is valid for arbitrary Q and K. The conditions for a determination of K from experimental data are discussed and compared to earlier analyses in limiting situations. The technique is more sensitive than cyclic voltammetry in cases of compressed potentials.