PdCuAu membranes are interesting tools for H-2 separation especially from sulfur contaminated mixtures. We have studied the preparation of such membranes on ceramic supports via electroless plating and examined their permeation behavior. Alloying of the separately deposited metals was time consuming at 500 degrees C under H-2. Hydrogen permeability improved and the associated activation energy became smaller with increasing Au and decreasing Cu content of the alloys. The low-temperature alpha/beta hydride miscibility gap was examined by in situ synchrotron radiation X-ray diffraction (SR-XRD) under H-2 employing PdCuAu samples alloyed at 800 degrees C under N-2. The gap narrowed with decreasing Pd amount and was reduced stronger by Au addition. This two-phase regime extended beyond 125 degrees C in hydrogenated Pd87Cu7Au6. Results from Cu-rich alloys indicate that it can be completely suppressed at room temperature through proper balance of Au and Cu in alloys containing less than 75% Pd. Hence, the risk of embrittlement due to formation of incommensurate alpha and beta hydride phases can be largely mitigated and Au addition to PdCu alloys benefits the low-temperature stability of such membranes. Moreover, the SR-XRD experiments under H-2 uncovered distinct alloys with very similar lattice parameters but differing hydrogen solubility in two Cu-rich samples demonstrating that such in situ studies are very useful for probing the homogeneity of multicomponent alloys.