This paper presents results of a laboratory study exploring the effect of the oxidation-reduction (redox) condition on crude-oil adsorption on rock mineral surfaces and consequent wettability alteration. Experiments were performed by aging Berea outcrop and Loudon reservoir sandstones with crude oil under various redox conditions and water saturations. Wettability alterations after aging were evaluated by the U.S. Bureau of Mines (USBM) method and by waterflood tests. The natural, oxidized Berea cores changed from strongly water-wet to mixed-wet conditions when aged at irreducible water saturation. Wettability alteration was greatly reduced when aging experiments were conducted under reduced conditions. The degree of wettability alteration depended on availability and redox state of iron on the mineral surfaces and also on aging time. We suggest two possible mechanisms to explain the role of iron in the wettability-alteration process. Wettability alteration in the water-wet Berea cores was insignificant when the cores were aged at residual oil saturation (ROS). Wettability alteration was also insignificant in the mixed-wettability Loudon reservoir cores, regardless of the water saturation or redox condition used in the aging experiments. We present the implications of these findings on core-handling procedures for wettability preservation.