A thermodynamic model is derived for the description of the equilibrium behavior of mixtures of a protein with an ionic surfactant adsorbed at the liquid/fluid interface. It is shown that experimental data for the mixtures HSA/CTAB (human serum albumin/cetyl trimethylammonium bromide) and beta-LG/SDS (beta-lactoglobulin/sodium dodecyl sulfate) agree satisfactorily with the theoretical estimates. The theoretical calculations of the adsorption behavior are based on the parameters of the individual protein and surfactant solutions. From the comparison between theoretical results and experimental data, quite reasonable estimates for the number of free charges in the protein molecule m were obtained. For the HSA molecule, m is between 5 and 10, and for the beta-LG molecule, m is between 2 and 3.