This communication summarizes key elements and facts of PdCu alloy membranes for H-2 separation by reviewing fundamentals and literature reports, such as the PdCu phase diagram and its implications toward membrane permeability and stability, the phase and composition segregation behavior, and the intricate phase structure-composition-stability-permeability relationship. The inconvenient challenges for applying PdCu alloy membranes due to its complex alloy phase diagram consisting of more than one phase structure are identified. The sulfur resistance and H-2 permeation hysteresis will also be discussed accordingly. Previously reported permeation data will be revisited by using the insights outlined to explain some of the previous observations of instability for PdCu alloys operating in pure H-2 atmosphere. We conclude the manuscript with a summary, as well as our perspective and outlook on this topic. (C) 2017 Elsevier B.V. All rights reserved.