An AC impedance technique in combination with a thin-film (12 μm) cell was employed to separately meusure the electrical resistance and capacitance of electrode-lubricant interfaces and bulk lubricant layers. In this study, three different additives were investigated by measuring the AC impedance spectrum of each additive mixed separately with a commercially available mineral oil. These additives were oleic acid, N-oleyl-1,3 diaminopropane (fatty amine), and mixed alkyl acid orthophosphute (acid phosphate). For lubricants containing 2 mass percent of an acid phosphate or fatty amine, the capacitance of the electrode-lubricant interface, Ci, and the capacitance and resistance of the bulk lubricant layer, Coand Ro, respectively, were evaluated separately from the AC impedance spectra. The lower limit of the resistance of the electrode-lubricant interface, Ri, was aDo estivnated. For additives, such as oleic acid, which physically adsorbs on the electrodes, the resistance and capacitance of the electrode-lubricant interface and the bulk lubricant layer could not be measured separately.Presented as an American Society of Lubrication Engineers paper at the ASME/ASLE Tribology Conference in Pittsburgh, Pennsylvania, October 20–22, 1986