The purpose of this study was to calibrate a dual-energy gamma system for simultaneous determination of aqueous and non-aqueous phase liquid (NAPL) saturations in soil columns. A dual-energy gamma system containing 241 Am and 137es was used to study the infiltration and displacement of aqueous liquid by NAPL and vis versa. Distilled water, NaI solution, and Soltrol 130 were used as fluids. The system was calibrated to minimize errors in liquid saturations due to the random nature of gamma photons emission and drift in the photon detection equipments. The measurement accuracy in liquid saturations was substantially improved by increasing counting time and fluid attenuation coefficient. NaI salt was used to increase the attenuation coefficient of water. Measured and predicted liquid saturations during immiscible displacement in soil were consistent with mass balance measurements and anticipated deviations in saturations from probable error calculations. The calibration procedure resulted in a significant improvement in the prediction of liquid saturations using dual-energy gamma system.