Physical vapor deposited Al/Ru bi-layers on silicon substrates have been annealed to study reactions development. Temperature induced changes after increasing time from 10 to 2880 min in vacuum annealing at 500 oC were studied. Grazing incidence X-ray Diffraction indicates RuAl2 phase formation in all samples. Electron diffraction pattern of a thin foil extracted from annealed bi-layers shows spots for RuAl2 and Al6Ru phases. Focused ion beam cross sections, shows non-uniform 500 nm thickness reaction layer at the Al/Ru interface. Decreasing thickness at a fixed ratio of Ru/Al = 1.224 reduces the time required to start reaction at the same temperature. Formed intermetallic phase layer acts as a diffusion barrier that controls further atomic diffusion from both Al and Ru sides into the formed reaction layer.