The efficiency of the 'ferrite process' for the purification of wastewater heavily contaminated with nickel is evaluated, and the solid residues formed are characterised. The efficiency of the purification process is always above 99.9% for Fe2+/Ni2+ ratios greater than 3. The tested Fe2+/Ni2+ molar ratios (15/1, 7/1 and 3/1) yielded three different nickel ferrites. Inductively-coupled plasma atomic emission spectroscopy (ICP-AES), potentiometric titration, X-ray fluorescence (XRF), X-ray diffractometry (XRD) and differential scanning calorimetry (DSC) yielded (NixFe1-xFe2O4)-Fe-II-O-III (x = 0.18, 0.40 and 0.65, respectively) as the most probable stoichiometry, and inverse spinel as the most probable structure. Heating at 600 degreesC causes the transformation of the solids into a mixture of NiFe2O4, alpha-Fe2O3 and NiO. Electrochemical analysis of the solid nickel ferrites was performed using carbon paste electrodes (CPEs) in HClO4 and HCl media. In each case, the First cyclic voltammogram showed the participation of solid species in the electrochemical transformation process, since the shape of the redox peaks could be related to the structure and stoichiometry of the ferrites. In second and successive scans. the voltammograms indicated the redox couples Fe-ads(3+) + 1e(-) double left right arrow Fe-ads(2+) (0.525 V vs. Ag/AgCl) and Ni-ads(2+) +2e(-) double left right arrow Ni(s) (-0.470 V) in HClO4, and FeCl2.ads+ + 1e(-) double left right arrow FeClads+ + Cl- (0.475 V) and NiClx.ads(x-2)- 2e(-) double left right arrow Ni(s) + xCl(-) (- 0.550 V) in HCl.