The present work gives a theoretical interpretation of the adsorption of cobalt on spherical colloidal hematite particles suspended in NaNO3 electrolyte, with the main stress on the dependence on ionic strength. The surface complexation model, as one very successful approach used over the past decade in predicting physicochemical phenomena at hydrolyzable oxide-electrolyte interfaces, was chosen. — On the basis of the Gouy-Chapman theory, different models of the electrical triple layer were applied by locating ions of the background electrolyte and cations interacting with surface OH groups at various planes in the layer. It was found that the position of the adsorption curve on the pH axis, its slope, as well as its practical independence of the ionic strength could be satisfactorily explained by a model with an FeOCo+ surface complex located at the "0" plane and all the others on the "β" plane.