We have used atomic-force microscopy to measure the change in interfacial friction and adhesion between a tungsten surface and bare or alumina-coated Si3N4 tips. These measurements have been made in aqueous solutions ranging in pH from 2.2 to 8.2 in the absence of chemical or mechanical dissolution of the surface oxide. Friction is observed to vary with solution pH, and the specific manner in which interfacial friction depends on pH is influenced by the composition of the interface. The maximum friction force of the tungsten/silicon nitride system occurs at a pH of 4.7. The tungstert/alumina system has a maximum in the frictional force at a pH of 6.6. Frictional variations in these systems are related to pH-mediated changes in the electrostatic interaction between the probe tip and the sample. The pH dependence of the electrostatic interaction is determined by the relative isoelectric points of the contacting surfaces. These nanometer-scale measurements demonstrate a fundamental pathway by which variations in the frictional properties of an interface are introduced by properties of the surrounding liquid environment.