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(111) surface. Pronounced changes in the photoelectron spectra due to UV-light exposure allow to calculate an effective cross section of σ eff ≈8×10 −22 cm −2 at 4.14eV and σ eff ≈4×10 −21 cm −2 at 4.4eV, respectively, for the trans- to cis-isomerization.