NiO scales thermally grown on pure nickel were characterised by in situ solid state electrochemical impedance spectroscopy (EIS). Possible sources of error including the amplitude of the applied sine wave signals, the input impedance of FRA, as well as the residence time at the measuring temperature were examined. Specimens oxidised in air for 6, 24 and 120 h at 800 degrees C respectively were characterised over the temperature range 150-700 degrees C. The results suggest that only one time constant charge transport process is dominant in the NiO scales. The temperature dependence of the electrical conductivity shows an activation energy of about 0.6 eV which is consistent with measurements on hulk NiO pellets. The capacitance and dielectric constants of NiO scales also show a strong dependence of temperature.