The surface force apparatus has been used to determine the interactions between hydrophobed mica surfaces immersed in aqueous solutions of the nonionic surfactant, dimethyldodecylphosphine oxide (DDPO) . The adsorption of surfactant on hydrophobic surfaces was studied at different concentrations. When a densely packed nonionic surfactant layer had adsorbed, the surface intetaction was measured as a function of temperature. Contact angles on hydrophobed mica sheets immersed in DDPO solutions were also recorded at different concentrations. The results show that strong, long-range hydrophobic attraction vanishes completely. In contrast, the adhesjion, as measured by the pull-off force, is substanially redused as soon as any polar groups are present on the hydrophobic surface. A repulsive hydration force is present between the nonionic DDPO head groups at short separations . There is an adhesive minimum between surfaces covered with DDPO monolayers. A temperature dependence of the interactions between the head groups was recorded and related to the temperature-induced phase separation present in the bulkDDpo.water system.