An atomic force microscope (AFM) was used to measure surface forces between a glass sphere and a flat fused silica plate under a number of conditions. Hydrophobic surfaces exhibiting contact angles ranging from 0 to 109° were prepared by reacting silica with octadecyltrichlorosilane (OTS). Contact angles, AFM images, and infrared transmission spectra showed that OTS forms clusters on the silica surface. The presence of water in the reaction was shown to greatly influence the formation of these clusters. Forces were measured between surfaces coated with identical (symmetric) and different (asymmetric) amounts of OTS to determine contributions from hydrophobic forces. The results showed that the hydrophobic force parameters of the asymmetric interactions, K132, can be predicted from those of the symmetric interactions, K131 and K232, using a geometric mean combining rule. Asymmetric force measurements were conducted between a hydrophobized glass sphere and a bare silica plate in dodecylamine hydrochloride (DARCl) solutions as a function of pH in an effort to simulate the forces involved in bubble-particle interactions for the quartz-amine flotation system. The appearance of the hydrophobic force in these measurements correlates well with the pH range of maximum flotation recoveries for quartz-amine flotation system.