Monolayers of saturated fatty acids of different chain lengths and of 9-cis-octadecenoic acid were physisorbed onto regenerated cellulose surfaces to study their effect on friction and adhesion. Lateral and normal force measurements on these surfaces were carried out at different humidities using an atomic force microscope with chemically functionalized silicon nitride cantilevers. Friction and adhesion of clean cellulose increased significantly with humidity. Adhesion forces at high humidity were predicted well by theory that accounted for the effect of the Laplace pressure due to capillary condensation. The magnitude of friction and adhesion and their dependence on humidity decreased with increasing hydrophobicity of the contacting surfaces. Friction of cellulose surfaces impregnated with different saturated fatty acids decreased sharply from a high level to a low level at a carbon chain length above 14 carbon atoms. Contact angle data, indicated that only saturated fatty acids with 16 carbon atoms or more in the carbon chain formed vertically oriented monolayers on cellulose. Our results suggest that friction of hydrophilic polymer surfaces under ambient conditions is greatly influenced by capillary condensation. Furthermore, lubrication by fatty acids is achieved through the formation of vertically oriented, hydrophobic monolayers that can withstand the normal and shear stresses during sliding and increase the hydrophobicity of the polymer surface, thereby decreasing the degree of capillary condensation.