Impedance spectroscopy (IS) was used to investigate the electrical properties of the lanthanum zirconate reaction layer which grows at the cathode-electrolyte interface of high temperature solid oxide fuel cells (SOFCs). The zirconate layer studied in this paper was obtained on the surface of a ZrO2-9.5 mol% Y2O3 Single crystal substrate by solid state reaction at 1500 degrees C between the zirconia crystal and a thin, e-beam evaporated, lanthanum oxide layer. The impedance spectra of the uncoated, single crystal substrate were compared and overlapped with those measured, at the same temperature, on the same sample with the reaction layer on one side. The electrical behaviour of the reaction layer was obtained as a function of the temperature (Arrhenius relationship) between 200 and 1000 degrees C, and compared with that of zirconia-based electrolytes.