The paper provides an overview of recent results of the radiotracer investigations of short-circuit diffusion in ultra fine grained (UFG) materials produced by severe plastic deformation (SPD). Different material classes (copper of different purity levels and Cu alloys) are considered. The study is focused on the existence of non-equilibrium grain boundaries after SPD. Although a dominant contribution of common high-angle grain boundaries with very similar diffusivities as those in the corresponding coarse-grained material is established, much faster diffusion rates are also observed experimentally. The nature and kinetic properties of these “high mobility” paths in different materials are investigated and critically discussed.