Hexadecyltrimethylammonium cations (CTA(+)) form ordered structures at the interface between muscovite mica and aqueous solution. The structure is altered in the sequence bilayer --> ordered cylinder disordered cylinder --> short cylinder --> sphere by successive addition of Cs+, that is, more curved structures are observed at higher salt concentrations. Parts of the same sequence occur at the interface on addition of other alkali cations, but the structure is more curved in the order Li+ < K+ < Cs+, following the softness of the ion. On addition of salt, the changes in aggregate shape at the interface occur in the opposite sequence to those that occur in bulk solution, demonstrating the dominance of surface interactions in this system. Substitution of a counterion that has a greater binding affinity for the CTA(+) ion, Br- for Cl-, leads to lower curvature structures at the same CTA+ and alkali cation concentrations. The effect of the counterion is similar to that observed in bulk, and is explained by a decrease in cation-cation repulsion in the presence of counterions that bind more strongly to the surfactant ion. The effect of rival cations can be explained by their binding affinity for the anionic mica substrate. The surface of muscovite mica has one negative charge per 0.48 nm(2). The high charge density induces a high density of surfactant cations, and thus low-curvature aggregates at the interface. When rival cations occupy these sites, the availability of surface counterions for the surfactant is reduced, and the aggregate curvature increases. The strength of this effect follows the softness of the ion, which we correlate with the relative binding affinity of the alkali metals for the mica/surfactant film compared to bulk water.