An improved method has been developed for the calculation of proton affinities of individual surface groups of metal (hydr)oxides, The method is a refinement of the multi site complexation (MUSIC) model, In the MUSIC model, Pauling bond valences are used to predict the individual proton affinities of surface groups of metal (hydr)oxides, One of the major changes in the refined MUSIC model is that the Pauling bond valences are redefined with a method in which the bond valence is related to the bond length. In this paper, the refined MUSIC model is applied to three different iron (hydr)oxides (goethite, lepidocrocite, and hematite). The resulting proton affinities can be combined with the crystal structure and morphology in order to describe the experimental charging curves. The charging curves could be described very well for minerals with a well-known (goethite) or reasonably well-known (lepidocrocite) crystal morphology, For crystals of which the morphology is less well known (hematite), the refined MUSIC model can be a powerful support for a suggested morphology,