The physical and chemical properties of zirconium chlorides have been studied in fused LiCl-KCl eutectic at 823 K using an electrochemical cell containing two platinum-oxygen electrodes with a solid electrolyte membrane. The first is used as pO(2-) indicator electrode and the second as an oxygen pump which allows us to vary the pO(2-) of the ionic melt by electrolysis. Titration of Zr(IV) chlorocomplex by O2-ions demonstrated the precipitation of ZrOCl2 and ZrO2. The solubility constants of these compounds were calculated. A Pourbaix type diagram, which summarizes the properties of zirconium species in the melt, was determined. (C) 2004 The Electrochemical Society.