The effects of adhesion, contact area, and pressure on the lubricating properties of self-assembled monolayers on steel have been investigated with friction force microscopy. The adsorbed molecules were fatty acids with varying degrees of unsaturation (0−2 double bonds; stearic, oleic, and linoleic acid) and a rosin acid (dehydroabietic acid), adsorbed from n-hexadecane solution. The friction of these loose-packed monolayers was studied in dry N₂ gas and in ethanol. Low adhesion (in ethanol) resulted in a linear increase in friction force at low loads, that is, F = μL, whereas higher adhesion (in N₂ gas) gave an apparent area-dependence at low loads of the form F = S_cA, where S_c is the critical shear stress. A recent model for the contact mechanics of a compliant elastic film confined between stiffer substrates was applied to the data obtained in dry N₂. Using this approach, we obtained interfacial energies of the compliant monolayers in good agreement with van der Waals−Lifshitz theory. With a low monolayer elastic modulus of= 0.2 GPa, we obtained a slightly higher value of S_c for stearic acid than that established for more close-packed stearic acid monolayers. An increase of μ and S_c was found with increasing degree of unsaturation of the fatty acid.