Three versions of a surface complexation model are described. The models differ in the assumed arrangement of ions near the surface and in the specification of the reaction with a surface site. The models were compared in terms of their comprehensiveness, their inherent realism, their predictiveness, and their ability to model the adsorption of phosphate, fluoride, sulfate, and silicate. A model in which the ions were allocated to a single plane described adsorption fairly well under restricted conditions. However, it was not comprehensive as it could not describe the adsorption of a range of ions in a consistent way. It was also unrealistically simple. A model in which the ions were distributed between three planes failed when adsorption approached a maximum. This was because it had no effective way of describing differences in maximum adsorption reached by different ions. In this respect, it was unrealistic. It also failed to describe adsorption of fluoride ions. This shows that the mechanism postulated by this model to explain the behavior of divalent ions could not be extended to monovalent ions. This lack of comprehensiveness suggests that the mechanism may not be valid. A model in which the ions were allocated to four planes was very effective in describing adsorption. It was comprehensive in that it applied to all ions in a consistent way. It was realistic in that it permitted the adsorbed ions to reside in a separate plane and also permitted the maximum adsorption of these ions to differ from each other and to differ from that of protons. With this model, adsorption of anions derived from polybasic acids could be described by assuming that only one of the ion species present reacted with the surface. For the inorganic acids it was the divalent species. This model was therefore predictive in that it permitted the different effects of pH on adsorption to be explained. As the pH is increased, the electric potential of the surface becomes less favorable for adsorption of anions. However, below the relevant pK, this is opposed by an increasing proportion of the reacting ionic species. This becomes less important as the relevant pK is approached and consequently causes an inflection point in curves of adsorption versus pH near the pK.