Single-crystal TiN(001) layers, 60 nm thick, were grown on MgO(001) by ultra-high vacuum (UHV) reactive magnetron sputter deposition at T-s = 700 degreesC in pure N-2. Epitaxial Al(001) overlayers, 160 nm thick, were then deposited at T-s = 100 degreesC without breaking vacuum. Changes in bilayer sheet resistance R-s were monitored continuously as a function of time t(a) and temperature T-a during annealing while changes in bilayer microstructure and interfacial reaction pathways were characterized using transmission electron microscopy (TEM) and X-ray diffraction following selected annealing times and temperatures. The combined results indicate that bilayer reaction is initiated at the Al/TiN interface with the formation of a thin continuous epitaxial AlN(001) interfacial layer in a metastable zincblende structure. Ti atoms, released upon formation of AIN, diffuse to the Al overlayer leading to the nucleation and growth of an epitaxial tetragonal Al3Ti layer. Reaction kinetics obtained from R-s(T-a,t(a)) curves together with TEM observations of reaction pathways indicate that growth of both the AIN and Al3Ti layers is limited by diffusion of reactants, with an activation energy of 2.3 +/-0.1 eV, through the AIN blocking layer.