A new model has been formulated for the electrochemical instability at the liquid vertical bar liquid interface, for which only a rudimentary model [T. Kakiuchi, J. Electroanal. Chem. 536 (2002) 63-69] had been available. The new model considers the diffuse double layers in the presence of the specifically adsorbed surface-active ions, whose adsorption Gibbs energy linearly varies with the phase-boundary potential. The electrocapillary equation in the presence of the common ions partitioning in both phases is employed under the assumption that the partition of the surface-active ions is described by the Nernst equation. The excess surface charge due to the specific adsorption of surface-active ions is allowed for in calculating the structure of the diffuse part of the electrical double layers on both sides of the interface. The calculated electrocapillary curves agree with the experimental results. It is confirmed that electrocapillary curves can have positive curvature in a certain potential region, which signifies the presence of an instability window. (C) 2010 Elsevier B.V. All rights reserved.