Within the framework of a system of two mixed Frumkin isotherms, the shape of the Gibbs energy surface for an adsorption layer formed by two coadsorbed organic substances is analyzed as a function of their degrees of surface coverage theta(1), and theta(2). It is shown that, for displacement coefficients n(1) = n(2) = 1, an increase in the parameter of intermolecular interaction a(12) results in a decrease in the number of possible solutions of the aforementioned system of equations. At the same time, for n(1) = n(2) = 2, there exists a region of a(12) parameters, in which the system of equations can have seven solutions, so that the number of possible solutions passes through a maximum with an increase in a(12). The results obtained are important for simulating unstable states, which can be realized on the electrode \ solution interface during coadsorption of two different organic substances. (C) 2003 Elsevier Science B.V. All rights reserved.