The purpose of this work was to investigate the functional dependence of the adhesion (pull-off) force on probe radius in AFM force measurements. Pull-off forces for hydrophobic AFM probes in contact with hydrophobic substrates were measured in water. Chemical functionalization of probes and substrates was accomplished by self-assembly of octadecanethiol on Au. Average pull-off forces for 10 SAM/Au-coated probes were measured and found to depend linearly on the probe tip radii, which were determined by scanning electron microscopy (SEM). Using the JKR contact mechanics model, the thermodynamic work of adhesion (Wad) determined from the pull-off forces was 110±10 mJ/m2 which compares favorably with estimates of Wad based on reported interfacial energies. These results verify the linear dependence of the adhesive force on probe radius, as expected from contact mechanics models. They also verify that SEM can provide self-consistent and reproducible estimates of the probe radius.