ABSTRACT Most rocks ore normally water-wet. However, If the rock has been in contact with crude oil, the coating of the grains with asphaltenes and resins may change the wettability from water- to oil-wet. This change cannot be overlooked since wettability affects such properties as capillary pressure, relative permeability, and electrical resistivity. Besides its effect on formation properties, wettability plays an important role in recovery mechanisms. The technique developed previously for in situ wettability determination at the oil-water contact level has been extended to the movable oil-residual oil contact level. Some formations contain a residual oil saturation below the movable oil level due either to a gradual escape of the hydrocarbons through the upper confining layer during geological time, or to efficient water encroachment. Experiments conducted in a water-wet sand pack have shown that, in the residual oil zone, the water pressure gradient is recorded up to the movable oil level where it changes to the oil gradient. The free water level is located below the gradient change and can be determined at the intersection of the oil and water gradients. In an oil-wet sand pack initially saturated with oil, the experiments show the pressure gradient changing from the water gradient to the oil gradient at the movable water level, which is located below the free water level. The shape of the pressure profile can be used to qualify wettability. The pressure data can also quantify wettability using the J-function method. Wettability determination in situ is possible with a formation pressure profile recorded with a wireline formation tester. An example is shown in which the contact angle is calculated. Formation pressure profiles recorded in the oil-water transition zone in wells with a residual oil saturation yield information on the wettability of the formation and the position of the movable oil level.