The spillover of hydrogen from Ni to Cu was directly observed on bimetallic Ni-Cu thin films and nanoparticles by hydrogen temperature programmed desorption (TPD) via depositing Cu atoms onto Ni surfaces precovered with atomic hydrogen. The spillover of hydrogen between metals could result in over counting of active sites by hydrogen selective adsorption methods, and also provides a new concept for catalyst design. (C) 2014 Elsevier B.V. All rights reserved.