Langmuir monolayers of arachidic acid have been prepared on a subphase containing uranyl ions (O=U=O)(2+). The interaction between the uranyl ions and the monolayer of arachidic acid has been studied by means of surface pressure and surface potential isotherms and Brewster angle microscopy at different pH values (ranging from 1.5 to 8). A similar systematic study has been carried out in the presence of Cd2+ ions for comparison purposes. The surface pressure and surface potential isotherms demonstrate that the acid to salt conversion of arachidic acid in the presence of uranyl ions occurs at lower bulk pH values (pH = 3.5) as compared with other bivalent metal counterions such as Cd2+. Changes in the surface morphology of the monolayer induced by the interaction with uranyl- and cadmium ions are analyzed and the correlation between the surface potential change and the morphology of the films is discussed.