A series of magnetic adsorbents (silica-magnetite composite oxides) containing 0, 20, 40, 60, 80, and 100 wt% magnetite were prepared via the sol-gel process, where 0.1-mu m-radius magnetite particles were encased in a mesoporous silica matrix. The experimental pH(pzc) for each of the pure oxides and their mixtures was tested against two applicable models in the literature; neither model explained the observed behavior. One of the models did not account for the heterogeneous nature of the two distinct surfaces present in these silica-magnetite composite oxides, and neither model accounted for the possibility of the silica locally altering the behavior of the magnetite functional groups. The behavior was corroborated independently, however, based on an analysis carried out with the measured surface acidity constants. This work also showed that the total exchange capacity of each of the mixed oxides was dominated by the high surface area of the silica, and that the electrolyte species, Na+ or NO3-, did not completely complex with the surface of the oxides, as expected for weak binding ions. The chemical component of the free energy of adsorption of protons for the acid-base reactions also dominated the solvation and coulombic contributions for each of the pure oxides and their mixtures.