The electrochemical oxidation of halides and reduction of halogens have been studied in sulfolane, an aprotic solvent. Stable trihalide ions X3-are formed. The formation constants K of these X3-ions have been determined by potentiometry as lo7.* (X = I), (X = Br), and lo3.' (X = C1) in a 0.1 M LiC10, medium. The values of the constants K have also been obtained in methanol, a hydroxylic solvent, lo4.' (X = I), (X = Br), and in mixtures of both solvents (X = I). The variations of solvation energies of X3-and X-ions from one solvent to the other were estimated by means of a ferrocene electrode.