Organic modification of clays with surfactants is required for the preparation of polymer-clay nanocomposites for a variety of applications. We have studied the structure and dynamics of interfaces in synthetic clays modified with phosphonium surfactants. The chemical shifts, line widths, and relaxation times measured by (31)p, C-13, and H-1 NMR and the relaxation times measured by impedance spectroscopy allow us to monitor the dynamics over a wide range of time scales. The results show that the phosphonium headgroup is most restricted and that the mobility increases with increasing separation from the clay surface. The carbon chemical shifts show that the 16-carbon and 12-carbon surfactant tails of hexadecyltributyl phosphonium and dodecytriphenyl phosphonium are disordered at the interface and experience mobility over a range of time scales. The dynamics depend most strongly on the structure of the surfactant headgroup, and tributylphosphoniums are more mobile than the triphenylphosphoniums. Two dimensional chemical shift anisotropy spin exchange experiments show that the phosphorus atoms in the triphenylphosphonium surfactant are immobile on the clay surface on a 1 s time scale. The dynamics measured by impedance spectroscopy show a similar dependence on headgroup structure, even though the processes occur on very different time scales and length scales. The relationship between the structure and dynamics of the interface and the properties of composites are considered.