A new experimental technique is described for simultaneous measurements of both the normal load and the transverse (frictional) forces between two molecularly smooth surfaces, their exact molecular contact area, their surface profile during sliding, and the distance between the two surfaces (to ±1 A ̊). Both mica and surfactant-coated surfaces have been used, and experiments were carried out in either controlled vapor atmospheres or with the surfaces immersed in various bulk liquids. We refer to the sliding of undamaged surfaces past each other at or very close to true molecular contact as "interfacial" sliding. At low loads the frictional force is described by the equation originally proposed by Bowden and Tabor: F = ScA, where A is the molecular contact area and Sc is the critical shear stress. The dependence of A on the load, L, is well described by the Johnston-Kendall-Roberts theory (for adhesive contacts) and the Hertz theory (for non-adhesive contacts) even during sliding. At higher loads there is an additional contribution to Sc such that the frictional force is now proportional to L. This contribution is analogous to Amontons' law, F = μL, but μ has a different origin and exists even in the absence of any interfacial adhesion. When damage occurs, Amontons' law is now applicable and μ is the normal coefficient of friction. The factors that determine the magnitudes of Sc and μ are different. These will be described, as will the two modes of sliding and the transition between them. © 1990.