Two-photon photoemission spectroscopy is employed to analyze reversible changes in the electronic structure of the molecular switch tetra-tert-butyl-azobenzene (TBA) adsorbed on Au(111), which are induced by UV-light and thermal activation. Cycles of illumination and annealing steps confirm the reversibility of the switching process, which we assign to a trans/cis-isomerization of TBA molecules in direct contact with the Au(1 11) surface. Pronounced changes in the photoelectron spectra due to UV-light exposure allow to calculate an effective cross section of sigma(eff) approximate to 8 x 10(-22) cm(-2) at. 4.14 eV and sigma(eff) approximate to 4 x 10(-21) cm(-2) at 4.4 eV, respectively, for the trans- to cis-isomerization. (c) 2007 Elsevier B.V. All rights reserved.