Diffusion-couple experiments were conducted on fine-grained alloys in the Al-Mg and Al-Zn systems. It is well known that grain refinement to the submicrometer range is feasible using the process of equal-channel angular pressing (ECAP). In addition, this process can be used also to produce microstructures with predominantly low-angle or high-angle grain boundaries by controlling the shearing direction and the extent of the imposed strain. Alloys having small grain sizes but different misorientation angles were used to make diffusion couples and the interdiffusion coefficients were determined by measuring the concentration profiles across the interfaces. The contribution of grain boundary diffusion to the overall diffusion was evaluated in terms of the alloying elements and the misorientation angles.