Potentiometric and cyclo-voltammetric studies have been carried out on monensin anion (Mon^–) complexes with the alkali ions as well as with Tl⁺ and Ag⁺ in absolute methanol solutions. The log K_f values obtained for the complexity constants and corrected for the activity effects are: Li⁺, 3.3±0.1; Na⁺, 6.72±0.05; K⁺, 5.18±0.05; Rb⁺, 4.58±0.05; Cs⁺, 3.75±0.05; Tl⁺, 5.31±0.05; Ag⁺, 8.2±0.2. It is seen that for the alkali, the most stable complex is formed with Na⁺. The enthalpy and entropy of complexation with the sodium ion were found to be H^o=–5.47±0.24 kcal-mole^–1 and S^o=+12.4±0.7 e.u. The complex, therefore, is enthalpy and entropy stabilized.