Abstract Because of unfavorable wetting conditions much residual oil is left when a porous material is flushed by water. Methods suggested to change reservoir wetting to improve oil displacement efficiency are generally expensive. The present laboratory study was undertaken to gain all understanding of the factors which determine reservoir wettability, and to find out if oil displacement efficiency might be improved by a wettability change accomplished at low cost in an oil reservoir. Contact angle measurements were made on mineral surfaces using several sets of reservoir oil and water samples. Results of the contact angle studies suggest that reservoir wettability may be primarily determined by natural surface-active substances present in the reservoir fluids. The effect of changing salinity and pH of the water phase was studied. The results suggest that gross changes in preferential wettability might be accomplished by injection of water containing simple chemicals to alter pH or salinity in the reservoir. Such treatment could be much less expensive than injection of commercial surface-active agents. Waterflood tests have also been made using synthetic cores and oil and water having wetting characteristics similar to those of reservoir fluids. Cores initially oil-wet were flooded in such a way that they were made preferentially water-wet by the advancing flood water. This reversal in preferential wettability achieved greater oil displacement efficiency than when either oil-wet or waterwet conditions were maintained throughout the flood. For the systems studied, the higher the oil viscosity the greater the percentage improvement obtained over conventional waterflood recovery. This suggests that a flooding process making use of wettability-reversal may extend the oil viscosity range over which water flooding is attractive. Because a precise adjustment of reservoir wettability does not seem to be required, and because altering the pH or salinity in some reservoirs may be inexpensive, it appears that a waterflooding process employing wettability- reversal could find successful field application.