Kinetic characteristics of internal interfaces in ultrafine grained (UFG) materials represent an aspect that is central to their potential applications. Interface diffusion is strongly affected by ultrafine crystallinity and incorporated defects. In the present study the radiotracer technique was applied for measuring grain boundary (GB) diffusion of Ni in UFG Cu-1wt.%Pb alloy. The UFG specimens were prepared by equal channel angular pressing at room temperature. GB diffusion was investigated at room temperature in Harrison's C kinetic regime under conditions in which Ni diffusion - even along general high-angle GBs in pure Cu - would hardly be measurable by the mechanical sectioning (as the corresponding penetration depths would be less than 100 nanometers). The present study highlights the existence of ultra-fast diffusion paths in severely deformed material. An atypical time dependence of the room-temperature diffusion data indicates a quite involved nature of these "fast" diffusion paths in the Cu-1wt.%Pb alloy.