The Langmuir-Blodgett (LB) technique has emerged as an alternative method to carry out a wide range of operations in different areas. LB films containing long-chain carboxylic acids and divalent metallic cations have important applications in the semiconductor industry, among others. In this paper, the chemical composition of LB films is predicted based on an electrochemical model that improves and completes previous efforts in this area. The model presented here constitutes an essential tool for the optimization of the experimental technique. Besides other features, the model predicts the concentration of the four species present at the interface (RH, R-, RM+ and R2M) and it allows a realistic calculation of the equilibrium constants (K-RM+ and K-R2M) for the formation of the complexes between the carboxylic acid and the cation (RM+ and R2M) based on a compilation of experimental data regarding the composition of the LB monolayers deposited on a solid substrate. In contrast with previous models, the present model does not constrain the possibility of the formation of complexes containing one and/or two RCOO-chains on the surface of the original Langmuir monolayer and it includes the non-ideal mixing behavior of the cations in the monolayer. LB composition data from eight cations (Pb2+, Cd2+, Ni2+, CO2+, Mg2+, Ca2+, Sr2+ and Ba2+) were used in order to obtain their respective equilibrium constants. The values obtained for these constants are lower than those typically reported, which are those of short-chain carboxylic acids in solution. The model also predicts the phenomena of charge reversion on the Langmuir monolayer for certain cations at high bulk concentrations and points to possible solutions to the unwelcome X-type LB depositions based on the values of the surface electric potential. (c) 2007 Elsevier B.V. All rights reserved.