Self-diffusion experiments based on neutron reflectometry and isotope multilayers in ultrafine-grained copper have been reported. The capability of this method to determine diffusion lengths in the order of 1 nm enables the measurement of volume diffusivities in the type B kinetic regime identical to previously published data on single crystals. Such results are difficult to achieve by classical methods of diffusivity determination, such as the radiotracer method, due to limitations of the depth resolution.