Molecular processes in the frictional response of an alkanethiol monolayer, self-assembled on a Au(111) surface, are studied by means of high-resolution friction force microscopy in ultrahigh vacuum. With increasing load, three regimes are observed on defect-free domains of the monolayer: smooth sliding with negligible friction, regular molecular stick slip motion with increasing friction, and the onset of wear in the monolayer. Molecular contrast in the lateral force is found for inequivalent molecules within the unit cell of the c(4 x 2) superstructure. Significant differences in the frictional response are found between defect-free domains and areas including a domain boundary. Friction increases by an order of magnitude on domain boundaries in connection with irregular stick slip motion. This increased friction at domain boundaries is observed at loads below the onset of wear.