The sorption of lanthanum on hematite is studied in experiments for three different surface loading ratios. The experiments consist of following the evolution of the amount of cation sorbed on surface with pH, and as well as measuring the number of protons released during sorption for the three different surface loading conditions. The sorption edge shifts to high pH, and the number of protons released during sorption increases, when the surface loading increases. Three different surface complexation models (SCMs), with three different electrostatic descriptions of the interface, are used to fit the experimental sorption curves. The stoichiometries proposed by the models are compared with the measurement of the protons released. Descriptions of the interface are given by the diffuse layer model (DLM), the constant capacitance model (CCM), and a nonelectrostatic model (NEM). If the fit quality is comparable for the three models, only the electrostatic models are able to account for the dependence of stoichiometry on surface loading. On the other hand, the NEM gives the same stoichiometry, with the same number of protons released, when surface loading conditions change. This is not in agreement with experiment observations. The stoichiometries, confirmed by an independent experiment, and the value of the surface constants obtained are the same, error aside, for DLM and CCM for the three different surface loading ratios. The NEM gives different values, even if the fit quality is comparable.