Grain boundary diffusion of Fe in nanocluster-strengthened ferritic steel (Fe-14Cr-3W-0.4Ti-0.25Y 2 O 3 in wt.%) has been investigated. The steel was produced by mechanical alloying followed by hot extrusion. The final grain size was $200 nm. The diffusivity of Fe was measured within the temperature range 423-820 K. The grain boundary penetration at lower temperatures revealed a specific time dependence, which indicates a residual interconnected porosity in the ferritic steel. In order to quantify the percolating porosity, conventional radiotracer ( 59 Fe) diffusion measurements were combined with a study of room temperature penetration of liquid 110m Ag solution to distinguish between solid-state diffusion along boundaries and penetration along the surface of interconnected cavities. The presence of porosity affected the diffusion process, introducing a hierarchy of internal interfaces. The grain boundary diffusion coefficient and the diffusivity along internal surfaces were determined in the so-called type C-C, C-B and B-B kinetic regimes of interface diffusion in a hierarchical microstructure. Using the residual activity method and a 65 Zn tracer, the volume fraction of the percolating porosity was estimated to be 0.6%.