Adsorption of C2H2 molecules on a Ge(100) surface at room temperature have been studied by ultraviolet photoelectron spectroscopy (UPS) using synchrotron radiation. In the UPS spectra, molecular orbitals (MOs) of the adsorbed C2H2 as well as the Ge-C bonding states were identified. After annealing the C2H2-adsorbed surface causing partial desorption of the molecules, all the MOs were found to remain intact. In particular, the persistence of the pi(C-C) orbitals after partial desorption comprises firm spectro-scopic evidence against the existence of the so-called tetra-sigma-bonded structure of the adsorbed C2H2 molecules. (C) 2010 Elsevier B.V. All rights reserved.