The a.c. impedance of the LiNiO 2 electrode was measured in organic electrolyte solutions based on mixed carbonate solvents containing inorganic lithium salts. The electrolyte composition influenced the impedance response of the oxide electrode as well as the charge and discharge characteristics under constant-current polarization conditions. The impedance responses changed with the state of the charge of the oxide, i.e. the x value in Li 1− x NiO 2. The impedance diagrams were analyzed using three equivalent circuit models. The interfacial resistance and the apparent diffusion coefficient of lithium, evaluated from the impedance responses, varied with the sort of solvent and salt. The interfacial resistance showed a minimum value around x=0.5 in Li 1− x NiO 2 for every electrolyte system, while the apparent diffusion coefficient decreased with decreasing x value. The relation between the electrolyte composition and the impedance response of the oxide is discussed. The electrolyte composition influences the effective cross section of the diffusion process in the solid phase through its effect on the surface chemistry of the oxide cathode.