Two different theories based on a multiple equilibrium model for analysing the binding data for ionic surfactant-protein interactions are investigated and modified, and intrinsic and statistical Gibbs free energies of binding per mole of surfactant are estimated. The characterization of the two models and interpretation of the binding process in terms of intrinsic and statistical binding free energies are discussed. These theories are applied to analysis of sodium n-dodecyl sulfate binding to ribonuclease A and lysozyme.