Among several different approaches to form Pd/Ag alloys for hydrogen separation applications, ex situ studies carried out by conventional X-ray point scanning detectors might fail to reveal the key aspects of the phase transformation between Pd and Ag metals. In this respect, in situ time-resolved high-temperature X-ray diffraction (HTXRD) was employed to study the Pd/Ag alloy phase nucleation and growth kinetics. By the use of linear position sensitive detectors, advanced optics and profile fitting with the use of JADE-6.5 software, isothermal phase evolution of the Pd/Ag alloy at 500 degrees C, 550 degrees C and 600 degrees C under hydrogen atmosphere were quantified to elucidate the mechanistic details of the Pd/Ag alloy phase nucleation and growth pattern. Analysis of the HTXRD data by the Avrami model indicated that the nucleation of the Pd/Ag alloy phase was instantaneous where the growth mechanism was through diffusion-controlled one-dimensional thickening of the Pd/Ag alloy layer. The value of the Avrami exponent, n, was found to increase with temperature with the values of 0.34, 0.39 and 0.67 at 500 degrees C, 550 degrees C and 600 degrees C, respectively. In addition, parabolic rate law analysis suggested that the nucleation of the Pd/Ag alloy phase was through a heterogeneous nucleation mode, in which the nucleation sites were defined as the non-equilibrium defects. Indeed, the cross-sectional SEI micrographs indicated that the Pd/Ag alloy phase growth was strongly dependent upon the deposition morphology of the as-synthesized Pd and Ag layers formed by the electroless plating. Based on the Avrami model and the parabolic rate law, the estimated activation energies for the phase transformation were 236.5 kJ/mol and 185.6 kJ/mol and in excellent agreement with the literature values (183-239.5 kJ/mol). Finally, the in situ annealing of the 15.6 mu m thick composite Pd/Ag/PSS membrane at 550 degrees C in hydrogen atmosphere indicated that the Pd/Ag alloy phase formation was not complete even after 500 h. According to the Avrami model, the increase in the hydrogen permeance from 7.1 m(3)/m(2) h atm(0.5) to 21.3 m(3)/m(2) h atm(0.5) at 550 degrees C over a period of 500 h corresponded to an similar to 83% Pd/Ag alloy phase formation. (C) 2007 Elsevier B.V. All rights reserved.