A few hundred ppm of ethylene glycol (a major component in antifreeze) can cause detrimental changes m engine oil. In this work, the a.c. impedance technique has been employed to measure the bulk-layer resistance of engine oil containing glycol. It is found that the bulk-layer resistance declines abruptly as the glycol concentration increases from 50 to 150 ppm. In addition, the bulk-layer resistance for glycol-contaminated engine oil remains constant during a 10-day time span, indicating that the measured resistance is not affected by the formation of micelles between glycol and detergents/dispersants. However, commercial engine oils contain different amounts of detergents, and the bulk-layer resistance for fresh oils varies. Therefore, a software program is necessary to compare the bulk-layer resistance measured from fresh and glycol-contaminated engine oil in order to determine precisely the extent of glycol contamination.