A new technique is described for directly measuring the capillary pressure required to rupture brine films on mineral surfaces. These measurements provide direct evidence for the creation of mixed-wettability states in oil reservoirs. In addition, the measurement of the critical disjoining pressure allows us to observe systematic variations in wettability as the capillary pressure is increased. The effect of brine salinity, pH, surface charge, and surface curvature have been quantified. Results for two reservoirs, one from Texas (Moutray) and another from Alaska (Prudhoe Bay), are presented.