Dissociative adsorption of H 2 (D 2) on Au thin films grown on an Ir{1 1 1} surface has been studied with temperature-programmed desorption using a quadrupole mass spectrometer and nuclear reaction analysis. Thin Au{1 1 1} films are epitaxially grown on Ir{1 1 1}, as confirmed by low-energy electron diffraction and scanning tunneling microscopy. H 2 (D 2) was dissociatively adsorbed on these Au{1 1 1} films, although it is well known that Au surfaces are noble enough not to dissociate hydrogen molecules. We propose a model to explain the unexpected high reactivity of the thin Au{1 1 1} surface, where narrowing of the s-band is responsible. Moreover, we found that H (D) atoms can be confined into the interface between the Ir surface and the Au thin film. We demonstrate how reactive Au thin films grown on an Ir surface are, and also compare the Au films with Ag films grown on Ir in order to test the validity/generality of our proposed model.