Abstract <p>The stability constants of the mononuclear, binuclear, and protonated copper(II) complexes with cryptand[2.2.2] in aqueous and aqueous dimethyl sulfoxide solutions were determined by potentiometric titration at <i>T</i> = 298 K and µ → 0. It was established that an increase in the dimethyl sulfoxide content in solution leads to decreased stability of the mononuclear and protonated copper(II) cryptates and increased stability of the binuclear copper(II) cryptate complex. The Gibbs energy for the transfer of copper(II) cryptates from water to the water–dimethyl sulfoxide solvent was calculated using our own and literature data. The solvation contributions of reagents to the change in the Gibbs energy of the reactions were evaluated.</p>