The fate and transport of metal ions in the environment is often controlled by sorption to clays and clay minerals. Substantial research has been directed toward developing mechanistic models capable of describing sorption reaction phenomena over a range of field-scale behavior. These efforts have led to the development of modeling approaches that attempt to depict the reaction of solutes with mineral surfaces using molecular spectroscopy. In this research, macroscopic data for divalent metal ion sorption to mineral oxides were used in conjunction with X-ray absorption spectroscopy (XAS) data to estimate surface complexation model (SCM) parameters. The XAS results were used to identify the types of surface complexes formed in single and bi-solute systems. SCM reactions were selected based on the XAS results, and metal ion binding constants were determined for the single-solute systems. These constants were then used to predict sorption in bi-solute systems. The modeling results show that SCMs often fail to predict bi-solute sorption.