Molecular modeling techniques are used to investigate the interaction of alkyldiphosphonate molecular anions with the surfaces of barium sulfate. We show that the most stable sites are on the (100) and (011) surfaces with the [PO3](2-) groups of the diphosphonates replacing surface sulfate ions. These results are compared with experimental observations; and analysis of the calculated energies demonstrates that the overall binding energy represents a subtle balance between the internal energy of the diphosphonate and the binding energy of the distorted diphosphonate in the site.