Hydrogen selective palladium membranes are important for technological processes of H separation and purification, but they become brittle because of repeated alpha - beta phase transformations. Pd is usually alloyed with Ag to avoid this membrane embrittlement, assuming a single-phase behaviour during hydrogenation. However, in the previous experiment 5-25-247 on D2B, in-situ measurements on Pd0.77Ag0.23D x at variable T and p(D2) revealed a transformation between two FCC phases with different lattice constants and deuterium contents, depending on time and high-pressure pre-treatments. The D-poorer phase seems to be metastable and to transform progressively into the D-richer one. An experiment is thus proposed on the high-resolution high-intensity D20 diffractometer, with the aim of studying the kinetics of this process. Two days are requested, to collect series of 20 consecutive patterns at 80 and 160 °C, and pressures in the 1 to 12 bar range, including the study of baric history effects.