The wetting properties of crude oils are measured using a new technique that measures the capillary pressure required to rupture thin films of brine on mineral substrates. An Atomic Force Microscope is used to record the force required to rupture thin brine films on quartz surfaces. Low values of critical capillary pressure imply that large sections of the reservoir may be rendered mixed-wet because the brine films are unstable. High critical capillary pressures imply water-wet reservoirs. The technique also provides a direct indication of the height above the water table, i.e., the capillary pressure at which mixed-wet conditions may be expected in the reservoir. Data presented for a pure hydrocarbon (octadecane) and a Prudhoe Bay crude oil indicate that the critical capillary pressure varies with pH, salinity oil composition and mineralogy. The pH and salinity dependence is explored in some detail in this paper.