In this work, time-resolved, in situ high-temperature X-ray diffraction was used to study the solid-state transformation kinetics of the formation of the fcc Pd/Cu alloy from Pd/Cu bilayers for the purpose of fabricating sulfur-tolerant Pd/Cu membranes for H-2 separation. Thin layers of Pd and Cu (total similar to 15 wt % Cu) were deposited on porous stainless steel with the electroless deposition method and annealed in H-2 at 500, 550, and 600 degrees C. The kinetics of the annealing process was successfully described by the Avrami nucleation and growth model, showing that the annealing process was diffusion controlled and one dimensional. The activation energy for the solid-state transformation was 175 kJ/mol, which was similar to the activation energy of Pd-Cu bulk interdiffusion. Furthermore, the Avrami model was able to successfully describe the changes in permeance and activation energy observed in Pd/Cu alloy membranes during characterization as they were annealed at high temperatures. (C) 2010 American Institute of Chemical Engineers AIChE J, 56: 3062-3073, 2010