Abstract Interest in the copper halides arises from their superionic or fast-ion conducting properties, i.e., the conductivity in the high-temperature solid is comparable to that in the melt. A further interest is that the changes are accompanied by structural phase changes. A considerable amount of structural work has shown that the halogen anions are closely bound to lattice sites and the cations are relatively mobile. Quasielastic neutron scattering has been used to probe the dynamics of both CuCl and CuI to confirm any diffusion. The results on both systems are compared with recent molecular dynamics simulations.