Аннотация:The simultaneous adsorption of guanidinium cations and ClO//4** minus anions has been studied. The experimental data indicates an absence of two-dimensional condensation, in contrast to the case of simultaneous adsorption of guanidinium and nitrate ions. The gain in free energy of adsorption on going from the solution air interface to the uncharged mercury surface is 6. 9 kJ/mole with DELTA sigma equals 0. 8 dyn/cm. This is close to the analogous value for guanidinium nitrate; however, the size of the attraction constant in perchlorate is lower than in nitrate. It is shown that the phenomenon of two-dimensional condensation is only possible with a sufficiently strong interaction between the cations and anions in the surface layer.