We demonstrate sub-surface absorption of hydrogen on Cu(1 1 1) to be energetically favoured, even in the limit of zero coverage and without surface reconstruction. Helium atom scattering measurements indicate desorption of hydrogen from the surface at a lower temperature, than that required to remove all absorbed hydrogen. The stablest bonding site, which is invisible to the helium beam, is therefore not a surface site. Our experimental observations of the behaviour of Cu(1 1 1) differ from previous calculations and contrast with other systems, where population of subsurface sites generally occurs only after population of surface states, or after significant surface reconstruction. (C) 2003 Elsevier B.V. All rights reserved.