The frictional properties of heterogeneous organic materials have been investigated by atomic force microscopy. There is a good correlation between measurements of the friction coefficient in air and the cosines of the advancing water contact angles for mixed self-assembled monolayers composed of polar and nonpolar groups. The friction force is proportional to the surface free energy for these chemically heterogeneous materials. For mixed monolayers containing short and long chain thiols with methyl terminal groups there is a large increase in friction, attributed to alkyl chain disorder rather than a surface energy effect. There is a nonlinear relation between friction coefficient measurements in air and the cosines of the advancing water contact angles on plasma-treated polyester films. This is thought to reflect a contribution from the changed mechanical properties of the plasma-treated film surface in addition to the observed surface energy increase on these more complex materials.