The structure and characteristics of ferrite sludge obtained in the purification of waste water contaminated with metal ions, are dependent on the experimental conditions. Under optimum conditions, a ＇magnetic ferrite sludge＇ (mfs) is produced while variation of these conditions gives rise to a ＇non-magnetic ferrite sludge＇ (nmfs). Both are free of contaminants. DSC and X-ray analysis revealed that the mfs was composed of magnetite (Fe3O4). Heating at 200 degrees C causes the transformation of magnetite into maghemite (gamma-Fe2O3) and calcination at 700 degrees C gives rise to hematite (alpha-Fe2O3). The nmfs consists of magnetite and poorly crystallized goethite (alpha-FeO . OH) and undergoes similar changes with temperature. Electrochemical analysis of the mfs, the nmfs and the product obtained by heating mfs at a temperature above 700 degrees C (＇heated ferrite sludge＇, hfs) was performed using a carbon paste electrode in HCl and HClO4. A linear scan from the potential at I = 0 was performed in a positive direction and followed by successive cyclic scans towards negative potentials and back. The linear scan voltammogram (LSV) and the first cyclic scan voltammogram (CV1) indicate that solid species participate in the electrochemical mechanism of the chemical transformations. However, after the first cyclic scan the voltammograms (CV2 etc.) show that only dissolved species absorbed on the graphite (Fe3+, Fe2+, FeCl2+ and FeCl+) intervene.