A novel technique has been developed to detect the depletion of the alkaline reagent that occurs during the displacement of an acidified oil by an aqueous alkaline solution in a consolidated model porous medium. An acidobasic indicator (thymolphthalein) was added to the displacing aqueous solution in order to monitor the progress of the chemical reaction. The initially blue alkaline solution turned colorless after prolonged contact with the acidified oil, indicating that the alkali in the vicinity of the interface became depleted as a direct result of interfacial chemical reaction. Under such conditions, the displacement front changes to a nonreactive one, very similar to a pure water displacement. The disappearance of color was particularly pronounced at low alkali concentrations and low flowrates.