Friction force microscopy has been used to study single-component and mixed self-assembled monolayers of aminothiophenol and thiophenol on gold. The friction forces and transition pressures of mixed monolayers were intermediate to the ones of single-component monolayers, and varied systematically with composition. The strength of the adhesion was altered by working in dry N-2 gas or in ethanol. In all systems studied, low adhesion (in ethanol) resulted in a linear dependence of the friction on load already at low loads, whereas high adhesion (in dry N-2) gave an apparent area-dependence. However, for a given monolayer composition, similar transition pressures were observed in dry N-2 and in ethanol, suggesting that the overall monolayer structure was not strongly altered by the presence of ethanol. Similar observations were made for very close-packed monolayers of octadecanethiol.