Ion exchange equilibria of ultrathin Langmuir and Langmuir-Blodgett films of stearic and arachidic acids in contact with aqueous electrolyte solutions were studied experimentally and theoretically. A model considering electrochemical and thermodynamic aspects of the adsorption of protons, calcium, and cadmium ions from solution to the film has been developed. Key parameters are the binding constants (K’s) of ions and the mixing characteristics (or Flory-Huggins interaction parameters chi’s) of ions in the two-dimensional film lattice plane. The ternary system can be described by the parameter values determined with binary proton-calcium ion, proton-cadmium ion, and calcium-cadmium ion systems : K(Ca) = 2.9 x 10(2), K(Cd) = 8.6 x 10(3), K(H) = 3.6 x 10(6), chi(H-Ca) = -0.76, chi(H-Cd) = 0.13, and chi(Ca-Cd) = 1.0. Competitive ion adsorption was measured by FTIR ATR spectroscopy of either collapsed or ordered LB films. Data for the proton-calcium-cadmium ion system agreed well with the model predictions. The results show possible future applications of such thin films as ion sensors or ion exchange materials.