The effect of a commonly used modifier, isodecanol, on the cobalt(II) -chloride system with the tertiary diamine extractants N,N,N’ N’-tetra-n-octylethylenediamine (tetocen) and N,N-di-n-ocryl-2-(aminomethyl)pyridine (pyridioc) was studied. The diamine extractants behave as stronger bases, the extraction of cobalt(II) decreases and chelate extraction mode is enhanced as the alcohol concentration in benzene is increased. The observations have been discussed in terms of changes in the solubility parameter and dielectric constant of the diluent and the solvation energy and solubility of the solute.