The equilibrium between I and the product resulting from the addition of ethanol to one of its C=N double bonds : 3-ethoxy- 3,4- diphenyl- 1,2,5-thiadiazoline 1,1-dioxide (II), is studied by electrochemical methods in ethanol (EtOH)-acetonitrile (ACN) solvent mixtures. The equilibrium constant is measured and a complete mechanism for the electroreduction of I and II in ACN solutions containing EtOH is proposed. The voltammetric properties of the newly synthesized compound 3-ethoxy- 2- methyl- 3,4- diphenyl-1, 2,5-thiadiazoline 1,1-dioxide (IV) are also reported. A stabilization of the dianion of I by alkali-metal cations, such as K+, Na+ and Li+, and divalent cations (Sr2+ and Mg2+) is voltammetrically measured. Equilbrium constants are estimated and compared for the interaction of I2- and the alkali-metal cations in ACN solutions of I.