A model for treating the sorption of metal ions on hydrous metal oxides was established based on the assumptions that these materials are weakly acidic cation exchangers and have a discrete exchanger phase. The experimental results of the sorption of metal ions on the hydrous niobium(V) and tin(IV) oxides are found to be consistent with the formulas derived from the model by considering that the charge balance and the mass action law hold in the exchanger phase and cations are sorbed by the distribution between this phase and the bulk aqueous phase.