The paper presents an analysis of a variety of laboratory experiments substantiating the fact that caustic waterflooding can significantly improve waterflood recovery of certain low-gravity viscous crude oils. Success of the process depends on the presence of naturally occurring organic acids. Experiments show that caustic waterflooding can significantly increase oil recovery obtained before water breakthrough. Introduction Caustic injection as a method for improving waterflood oil recovery is not a new idea. U. S. Patent 1,651,311, covering waterflooding with sodium Patent 1,651,311, covering waterflooding with sodium hydroxide, was issued to H. Atkinson in Nov. 1927. There is no record of successful field application of the method described, however. In 1962 Leach et al. showed that caustic injection water could alter wettability and improve oil recovery in laboratory experiments, but the results of a small field trial were somewhat inconclusive. In 1970 Emery et al. showed that caustic injection could cause wettability reversal and improve the waterflood recovery of crude oil from the Singleton field, Nebraska, in laboratory experiments. A field trial of the process proved to be disappointing, however. The experiments described here show that there is an alternative to the wettability reversal mechanism by which caustic injection can significantly improve the recovery of certain crude oils. The mechanism involves the drastic reduction of oil-water interfacial tension by the caustic activation of potentially surface-active organic acids naturally occurring in the crude oil. The reduction of interfacial tension causes emulsification of crude oil in situ that tends to lower injected water mobility, damp the tendency toward viscous fingering, slow water channeling caused by reservoir stratification, and improve volumetric conformance or sweep efficiency. The laboratory caustic floods of viscous, lowgravity crude oils containing sufficient natural organic acids are characterized by improved recovery at water breakthrough and lower producing water-oil ratios (WOR). The mechanism involving lowered interfacial tension, in-situ emulsification, and water mobility reduction is supported by correlation of interfacial tension with recovery efficiency, observation of in-situ emulsification in thin, transparent glass bead packs accompanied by changes in areal sweep efficiency, and evidence that ultimate residual oil saturation or microscopic conformance in reservoir core material is not significantly affected by caustic injection. The process appears to have good economic potential for suitable crude oils. Sodium hydroxide is potential for suitable crude oils. Sodium hydroxide is an inexpensive material and most required concentrations for in-situ emusification range between 0.05 and 0.50 weight percent, about one-fifth the concentration usually specified for wettability reversal. Furthermore, slug injection of about 0.15 PV can sometimes be as effective as continuous injection in laboratory tests. Core Floods: Interfacial Tension and Caustic Concentration Core floods were carried out on preserved core samples using crude oil at reservoir temperature. Core samples were typically 3 in. long and 1 1/2 in. in diameter. Oil viscosity was adjusted to its reservoir value by addition of a few percent kerosene to compensate for loss of volatile components when necessary. JPT P. 1344