The sensitivity of ion-concentration distribution models to three key model assumptions, the pore-size distribution of clay media, the ‘distance of closest approach’ of ions to the clay surface, and the accessibility of sub-nanometer-wide clay mineral interlayer spaces to anions, was explored by solving the Poisson-Boltzmann equation for swelling and non- swelling clay materials. The calculations show that all three model assumptions impact significantly on values predicted for the anion-accessible porosity. As a consequence, macroscopic measurements of anion exclusion in clay media cannot be used to test any of the three model assumptions independently of the other two. Information gained at the nanoscale, a detailed characterization of pore-size distribution in particular, is necess- ary to develop accurate predictive models of the anion accessible porosity of clay media