[en] Using the solubility method second sphere complexing of tris-(ethylenediamine)-cobalt (3) with iodide-ions in isopiestic aqueous solutions of sodium perchlorate and iodide at the temperature 298.15+0.1 K and values of water activity: 0.96; 0.94; 0.92; 0.90; 0.88; 0.84; 0.76; 0.68, has been investigated. Stability constants of the complexes are calculated taking into account the effect of medium, proceeding from conditions of lgβ linear dependence on the concentration of iodide-ions. Extrapolation of concentrational constants results in the following values: lgβ₁⁰=1.4±0.1 and lgβ₂⁰=2.0±0.2