Interfacial tension has been used to study the adsorption at the water/1,2-dichloroethane interface of dicarbollylcobaltate(III) {(pi- (3)-1,2'B9C2H11)(2)Co(III)(-)} Of the monovalent and bivalent metals. At the limiting coverage, the area per dicarbollylcobaltate(III) anions in the monolayer found experimentally is equal to 95 +/- 8 Angstrom (2). It can be assumed that the symmetry axis of the adsorbed anions is arranged parallel to the interface surface. By their effects on the surface activity of the dicarbollylcobaltate(III) anions, cations can be arranged in the following order: Mg2+ approximate to Ba2+ approximate to Pb2+ greater than or equal to Li+ approximate to Na+ > K+ > NH4+ > Rb+ > Tl+ approximate to Ag+ approximate to Cs+. The parameters of Frumkin and the virial isotherms have been calculated. There is a rather strong repulsion among the adsorbed anions. To define the energy of the specific interaction between the anions and the adsorbed monolayer, the technique of E. Goddard et al. (J. Colloid Interface Sci. 24, 297 (1967)) was applied. Its application for two-phase systems permits us to define the parameters of the ion exchange reaction on the basis of measurements of interfacial tension. The ion exchange constants calculated by various methods have been compared.