The interfacial structure of muscovite in contact with aqueous CsI solutions was measured using surface X-ray diffraction for several CsI concentrations (2-1000 mM). At CsI concentrations up to 200 mM, Cs+ adsorption is likely hindered by H3O+, as both cations compete for the adsorption site above the muscovite hexagonal cavity. Above this concentration, more Cs+ adsorbs than is required to compensate the negatively charged muscovite surface, which means that coadsorption of an anion takes place. The I- anion does not coadsorb in an ordered manner. Moreover, the hydration ring and water layers do not change significantly as a function of the CsI concentration.