An electrically conductive polymer, poly(o-phenylenediamine) (PoPD), is soluble in dimethylsulfoxide (DMSO) without any pretreatment. Cyclic voltammograms of dissolved PoPD were measured in DMSO solutions containing halogenide ions and two reversible redox peak currents were evident. The redox potential shifted with the concentration of the dissolved halogenide ion. The relationship between the potential shift and the concentration determined the relative association constant of PoPD for four halogenide ions : 104 mol(-1) dm(-3) for F-; 32 mol(-1) dm(-3) for Cl-; 29 mol(-1) dm(-3) for Br- and 9 mol(-1) dm(-3) for I-.