A general methodology for investigating the mechanisms of deposition of ionic species containing catalytically active elements on the oxidic support/electrolytic solution interfaces has been developed. The methodology is based on the "two-pK/one-site" and "triple-layer" models (for the charging mechanism of the surface of the support, and for the electrical double layer developed between the surface of the support, and the impregnating solution, respectively) and exploits the experimental data of the adsorption isotherms at various pH of the impregnation suspension. Starting from a quite general, "postulated", deposition mechanism, comprising all equilibria that could possibly take place at the "support/impregnation solution" interface, the application of this methodology leads to the "proposed" deposition mechanism, namely, the set of equilibria that actually take place, the kind of deposited species formed, their relative concentrations, and their dependence from the impregnating parameters (pH, concentration of the precursor, ionic strength, and temperature). The derived equations, describing the deposition of a given ionic species on the support surface, show that the concentration of the deposited species depends on various factors among which the most important are the charge of the ionic species, its concentration in the bulk impregnating solution, the surface concentration of the receptor sites on the support surface, the pH, and the Galvani and Volta potentials developed, respectively, on the surface of the solid particles and at the inner Helmholtz plane. As a test for the validity of the proposed deposition model, the theoretical values of some "support/electrolyte interface" parameters (calculated on the basis of the deposition mechanism revealed by applying this methodology) are compared with the corresponding experimental values determined by potentiometric titrations and microelectrophoresis.