Affinity capillary electrophoresis (ACE) was used for investigation of solvent effect on stability constant of model complex of dibenzo-18-crown-6 with potassium ion in mixed hydro-organic solvents at different water/methanol and water/ethanol ratios. The stability constant of the complex was found to be strongly decreasing with increasing molar fraction of water in mixed solvents and almost independent of the organic solvent used.