Divalent metal ions binding to Langmuir-Blodgett (LB) films of behenic acid have been investigated over their complete binding pH ranges. The mass of ions incorporated into the LB films has been measured in-situ by a quartz crystal microbalance (QCM). The corresponding degree of ion binding to the precursor Langmuir monolayer (LM) has been calculated using the one-dimensional Cartesian Poisson-Boltzmann-Stern (PBS) approach, developed by Bloch and Yun,(1) allowing the two systems to be compared quantitatively. The results show that the degree of ion incorporation into an LB film is very similar to that of the precursor LM, even though the metal cations are in vastly differing environments in the two systems.