The adequacy of a molecular model to describe the adsorption of dodecyldiphenylphosphine oxide (DDPO) at a mercury electrode has been checked; the model explicitly treats the electrostatic and nonelectrostatic interactions in the adsorption layer on a molecular level and is based on a two-state adsorption isotherm derived by Sangaranarayanan and Rangarajan. Estimates of the adsorption parameters have been obtained on the basis of a method applying nonlinear regression analysis and an experimental sample for the decrease in the interfacial free energy with the addition of surfactant, as a function of electrode charge density and surfactant concentration. A critical discussion of the obtained parameters has been made. Finally an important restriction which should be imposed on adsorption parameters when applying the model has been elucidated and the peculiarities of the model have been shown if this restriction is not fulfilled.