The earlier developed theory of specific anion adsorption at the electrode/electrolyte interface has been adapted to describe the adsorption from electrolytes of constant ionic strength. The derived adsorption isotherm describes the influence of thermodynamic and molecular parameters (bulk concentration of surface-active anions, electrode surface charge, energy of specific anion interactions, anion diameter, mean solvation number of the adsorbed anion, and charge transfer effect) on anion adsorption at the interface. It is shown that properties of the diffuse layer are responsible for different anion adsorption from an electrolyte of constant ionic strength and from an electrolyte containing a single salt. Depending on the anion adsorbability, the specific anion adsorption from an electrolyte of constant ionic strength can be stronger or weaker than from that containing a single salt.