The interactions between deposited monolayers of dihexadecyl phosphate (DHP), immersed in aqueous NaCl solutions, have been studied. At pH 5-6 double-layer forces, well-described by Poisson-Boltzmann theory, were observed. The magnitude of the repulsive forces demonstrated that less than 2% of the molecules were charged. At small separations (D < 2 nm) attractive forces, which decreased with increasing NaCl concentration and pH, were present. At higher pH, a repulsive hydration force overcame the van der Waals force completely. Intralayer forces were investigated by recording pressure-area isotherms for DHP and for dioctadecyldimethylammonium bromide (DODABr). DHP formed a gaseous, a liquid condensed, and a solid condensed phase. When the pH or the NaCl concentration was increased, the liquid condensed phase became more important; thereby the area per molecule for a given surface pressure increased. We suggest that this is due to sodium ions replacing protons as counterions, which gives rise to an intralayer hydration force. The DODA monolayer behaved differently, and the area per molecule for a given surface pressure decreased when the NaCl concentration was increased. The same types of forces that operate between headgroups within a monolayer of DHP are also of importance for the interactions between such monolayers. The magnitude of some of these forces depends on the type of counterion.