The interactions between pairs of hydrophobic monolayers of dimethyldioctadecylammonium bromide (DMDOA) surfaces and pairs of octadecyltriethoxysilane (OTE) surfaces were measured directly using the surface force apparatus. In the case of DMDOA surfaces, a long-range attraction detected at about 100 nm was observed, in good agreement with published data for DMDOA. In contrast, for the OTE surfaces, no interaction was discernible until there was a jump-in at an estimated separation of 13 nm. Furthermore, for the DMDOA surfaces, cavitation was observed on separation, as seen previously, while for the OTE surfaces, cavitation occurred on contact (spontaneous cavitation). The nature or extent of interaction between the OTE surfaces did not change in the presence of electrolyte (up to 0.1 M KNO3). Adhesion force measurements for these surfaces in air and when immersed in water are presented.