The interactions between a stearic acid monolayer and different transition-metal ions (Cd2+, Mn2+, and Tb3+) have been investigated as a function of pH by measuring the compression and surface potential isotherms at the air/water interface. The degree of dissociation of the monolayers was determined for deposited monolayers by electron spectroscopy for chemical analysis (ESCA). The agreement between the results obtained for floating and deposited monolayers was good. A sharp decrease in the surface potential was observed close to the pH corresponding to a total conversion of the stearic acid monolayer to the corresponding soap. The investigated metal ions could be ordered in terms of increasing interaction with the monolayer as Mn2+ < Cd2+ much less than Tb3+. The divalent metal ions followed the order of increasing covalency of the interaction.